It seems the compost I used was more lively than anticipated. There's an earthworm crawling around in there.
It's a decent 12 inch pot, but it's still effectively a limited environment. So I'm not sure if I should leave him in or take him out.
UPDATE: alright. I'll keep him in, cross my fingers, and hope I've pulled in enough of an ecosystem to both sustain him for his expected lifespan, and safely handle the subsequent death and decomposition.
You can leave it in; it should help the plant out.
But be aware - many earthworms are actually super invasive species! They’re actually very bad for insect populations & such.