I am not sure but think it could be a liverworth, also known as greenhouse moss. They grow in moist environment, spread by spores.
I would get rid of it. I think they 'steal' nutrition from the plants, also spread & may smother tiny seedlings.

Rina

Thanks for the response. I plucked it out this morning. It had already started pushing up against the two nearest seedlings and the roots were just starting to connect as well. I was worried that opening the lid would cause such a drastic change in temperature that it might cause all of the seedlings to die, so I steamed up my bathroom and did it in there ;) Good? Bad? We'll see...