Yes, believe it's just slime mold, possibly some protein given off by the fish mixed in there- it won't harm anything in there, just unpleasant to the eyes. I have to wash all of the caves in my tanks frequently to remove the slime.
Mold eats things.. think of the mold on food.. will eat and grow. This is just eating something or other in the water - I haven't done much research on it, last time I looked into it was a little over a year ago. But never found a way to stop it other than washing it and sometimes when tanks mature it will become less and less.