MrStinkyanswered grow question 3 years ago Hmm, green indicates chlorophyll and that it is growing after being under a humidity dome could suggest 1 of 3 things:
Lichen
Moss
Mould
Most of these are harmless and indicate that the soil is too wet and can often appear if soil isnt allowed to dry properly between waterings.
Suggested course of action:
Remove green stuff
Allow too soil to dry properly after every watering and see if it comes back and if it comes back does the plant look ok or not?
If it looks ok then it is probably a harmless inhabitant of your soil.
It is possible that it came in your soil if you got something from a garden centre.