The biggest problem is that this is near unplayable in a group
Player races are meant to be played as player characters, and in D&D 5e, the assumption is that you adventure in a group of such characters. Drop
Alignment. Because of their state, and after being rejected by the rest of society, sporeborn will keep to themselves and either attempt, aimlessly, to find a cure for their condition, or wreak havoc amongst the mortal races in anger for their hopelessness, although some may find peace in their contact with nature.
This is entirely antisocial, and a non-starter for a game that assumes the character will be part of a mixed group. At least, unless all of them are sporeborn, and that is an unpractical assumption to make for a game where each player typically decides what character they want to play. Just drop this.
Monstrous. You are considered a terrible, detestable being by common folk, and must cover your body and smell to evade conflicts.
This likewise makes the race very hard to play in any normal campaign, where common folk are the backdrop of play, when you visit villages, towns and cities. Drow characters have had issus with this in past editions (when practially all drow were irrevocably evil). There are no straightforward ways to cover "smell", and the race has no feature to disguise itself. I'd remove the smell aspect, at least.
Power assessment
I will also take a look at the power level, and make some suggestions, using detect balance to try and get that race to a level that is in line with other races. One common trap that you fell into is making maybe too many special features, compared to typcial published races.
You should sum up to 25 points or so, and you already have 12 from the ability score increases. Height, age, speed and languages are standard and add or subtract nothing.
Fey: Clarify the language to "Intelligence (Nature)", if that is what you mean. Non-humanoid is +2, a skill is +2, total +4.
Blend In. Stealth is one of the most often used skills (based on statisitics the Critical Role team made), only a little behind the #1 skill perception, but here is limited to only within dense forests. Advantage on a situational roll is worth +2.
Flammable: drop this. Vulnerabilty is extremely hard to balance, because it just can spell death when being hit by a fireball or something like that, which normally would not be so deadly. Instead of using vulnerabilty to balance overtly strong other features, mellow down those other features. It would give you -20 points more to play with, but it really is a bad idea.
Spore Cloud: This needs to specify a DC, use language copying the published features, to be clear. You confirm in a comment the DC is 8 + Proficiency + Wisdom Ability Bonus, this should be in the text. It also should state that this causes the poisoned condition (again, clarified in comments), as there are many other kinds of poisons. The ability will also affect the rest of the characters, so has quite some risk to contribute to a TPK, as it might poison the whole team at a time when you already are down. It also does not state a duration. You could at least limit it to 1 round, to make it more bearable. This one is hard to score as it is as much a liabiltiy as a power, probably more so, unless the whole team is immune to poison (like with heroes feast). I'll put it at +0, and would recommend to cut this feature. You already get to spew spores with the Regurgitate one, so it is really a bit superfluous.
Immunity to poison: You added in comments the race also would be immune to poison, but this is not in the race description. Even simple resistance to poison is already +4 points, so immunity would be more than +8, but none of the published races has an immunity. You do not need this, so I would leave it off. If you use it, make it resistance at best. I'll assume you leave it off, for +0.
Photosynthesis: There is no limit on the hit dice you can take in a short rest. If what you mean is that you get one extra hit die to spend, in addition to your other hit dice, in most cases this is depending on your hit die and Con bonus worth between +d6-1 hp or +d12+5 hp. The average is likely somewhere around d8 or d10 + 2 or 3 hp, similar to a cure wounds, with the downside it only works during a rest, and worth about +3 points.
Natural: looks OK, it's a lot of free spells per day, and this is a level 3 spell. I would probably drop the level 10 rider, and limit this to once per long rest, starting at 5th level, which is +3 points.
Regurgitate. Assuming this uses the same DC as above. This also needs to specify the action cost -- I will assume this takes a full action. I think for consistency, you should really limit this to only the Constitution save, not an option of a Dexterity save instead. This is very strong, as it effectively the ability to give Disadvantge to 2 opponents nearly every fight. (There are typically 3-4 fights per day, so you can take a short rest in most cases in between). The only saving grace is that a lot of monsters have advantage against poison, or are outright immune to it. I think the ability to silence one creature may be too strong, it can entirely invalidate a spellcaster for a turn, and most fights are just 3 or so turns. I'd probably tone this down and reduce the range to 5 or 10 feet so you need to at least get close to that spellcaster, and they often seek protection behind some meatshields. I'd suggest to drop the part of silence affecting this, first spewing spores is not making noise, so silence should not affect it, second, it is very rare that PCs get silenced, so it is just rules overhead that needs to be remembered for little play time. A comparator is ray of sickness, which also causes the poisoned condition on a failed save for 1 turn, has a range of 60 and is a first level spell that costs an action (it also requires you to hit first). So this is worth at least 3-4 times the 3 points a first level spell costs, and this is arguably twice as strong with hitting 2 targets and not needing a to-hit roll, and being able to silence spellcasters, so it probably should be at least level 2. This would be worth far more than 12 points.
At this point, you are already at 12 + 4 + 2 + 3 + 3 = 24 points, so you only really can afford 1 more. Make this a daily power, maybe comparable to a level 2 spell available from 3rd level, which would be 3 points, getting you to 27. That is already on the higher side, and if we misevaluate something, we might be over the top, so you could consider to drop something, either Blend In, or maybe Intelligence (Nature) or another of the +2 or +3 features, depending on what is most important to you.
