OK, so we don't have a picture because I had Owen wash them all off the outside of the compost bin before we researched it, but it turns out the largish larvae on our compost bin are probably not leeches or anything scary. We think they are the larvae of the black soldier fly, often bought to feed to pets or to help break down compost in bins. Wow, and to think, we got them for free . . .
It's pretty gross being green sometimes, especially when the adults of this lovely larvae are really annoying and weird looking pests we get in our windows all the time. But I will say, our compost is doing really well!