Honestly I don't know what that is. There are sea slugs that eat leathers but they are not black. The pic in post #146 looks like there is one off the lether on the rock to the right. Any way you can remove that rock to get a closer look? If I had to guess, I would still point to a sponge but it's odd that they just popped up. I'll keep looking and if I come across something I'll let you know.