Feeding sea cucumbers that have wedged themselves into the rocks? Reminds me of these guys: http/www.divegallery.com/sea_cucumber.htm but I don't know my cucumber species well enough to know if there's a black variety like that.
EDIT: first link was a dud, so fixed it hopefully.
Erm I think its two separate things ....
Im pretty sure there was only one last week then another appeared, but I did notice a slug looking thing in the same location which was cyclinder shaped and about a inch in length ...
Just hope it does multiply whatever it is