It is certainly not possible for a plant to survive on any of our asteroids, even on Ceres the conditions are completely unsuitable (moulds, by the way, are fungi, not plants, and couldn't survive unless there were plants present). The other obstacle to plants emerging on a Ceres-sized asteroid is that Ceres never had suitable conditions for life to emerge.
I think Pluto or Eris is about as large as an asteroid could get and still qualify as an asteroid, and there probably are solar systems with asteroids that size within the habitable zone, in which case it is conceivable that life might arise on them. Unfortunately they would be likely to end up like Mars, which once had conditions suitable for life, but lost most of its atmosphere over the course of billions of years. Micro-organisms might survive on such asteroids, but nothing more advanced.