Thanks for the replies everyone. I'm glad people seem to be leaning more towards some sort of bug that got in the cage rather than a parasite. I was leaning that way too, since it didn't seem to be in the poo.

I do not think they are maggots. Why would maggots be crawling up the plastic instead of going for the poo? And they just don't quite look like maggots.

Something I should have added in the first post is they don't move. So they are either dead, something that moves slowly & rarely, or some sort of egg case.