ok - it is sponge. I knew all along to be honest. I just wanted to get at least someone to tell me I'm a mental. You're all too polite.
It stayed for three frames, then disappeared. It's on my new-gen holga body, I've never had it with the old holgas so I'm guessing it's a foam quality issue. I would say it is funny, and put it to one side, but I walked for an hour to get to the Mausoleum, spent some time snapping with various cameras, then walked back only to find that I'd dropped the metal lens cap to my CV 50mm nokton somewhere along the way. So I had to walk all the way back to look for it. I did find it, right by the mausoleum, so I walked for 4 hours in total to get foam-ruined negs. bah.