I think I know what you are talking about. I suddenly noticed the same kind of stuff in my six weeks old 20g saltwater tank, this morning. My wrasse was stuck in it. Therefore in answer to your question if it could hurt the fish? answer is yes (especially if it is the same thing as I had this morning). Be careful!!!
I immediately did a 15% water change in my tank and fished out these things with a net. Haven't seen any more of them since. I have posted my questions as to what are these things and how to avoid them in future, in sw section.