Have the hp gain trigger on the permanent's turn after it tries to drain quanta.
This means creatures must survive their first player turn (not creature turn) before they get the benefit.
I get what you're saying, but would this require new text in the card or just a bit in the extras text box in the table?
Nothing new needed. However "Mycelium gives each creature on your field 2 HP per turn. Drain
each turn." would be an improvement.
As in one permanent buff or a buff a turn?
EDIT: Oh yeah, one-thousandth post!
"per turn" would be a buff per turn. Remove the "per turn" if you want a renewed one time buff. (I am unsure which you are intending)
Be careful not to underestimate the absorb cost.

per +2hp is not worth it.