Seems like an interesting creature, on the one hand it could be compared to firefly queen or pharaoh in that it is generating a daughter creature and creating a large swarm. but unlike those cards, it also seems to be a sort of power increasing creature akin to a golem or forest spirit, but mono element. Capable of remaining as separate creatures, or merging into a single powerful mass.
Honestly I'm not entirely sure what sort of power I would say it has, but it might be a tad UP due to the cost of the ability, as Rutarete said, perhaps change it to
instead of
.
Actually, due to the fungus creature generating life quanta, leave the ability cost as it is. tis the equvilent of having it at
anyway, because are you going to WANT to not create another creature? you can't even point out creature limits as an issue with that since you can just merge one into the whole to make room.
Which brings up a thought, this is rather powerful against CC cards like antimatter and liquid shadow, as in the former case you can just merge enough of the creatures into the single creature that the antimatter is negated, and in the latter case, give it more health to combat the poison. keeping them seperate for that purpose brings up a weakness to fire storm and thunderstorm, but most wouldn't have either of the first two and also have the latter two, in short the card can actually adapt to what sort of CC your opponent has. Grow it a bit so that a lightning bolt won't destroy the monster in one hit, and leave a few out to counteract antimatter, poison and the like.
Despite the turn delay on the growth, it's actually not that far behind creatures such as lava golem and forest spirit if you want to simply grow one large creature, as you can make a new one on the same turn as you merge an old one back into it. it's just a bit behind. The fact that it is mono color is also plus.
TL;DR
Overall, while there may be a kneejerk reaction to call it UP, I think it may actually be fairly balanced or even OP due to its versatility and mono color. leaning more towards the balanced though