Toothbrush & Elbow Grease
Looks like Hair algae, fairly easily removed with a toothbrush, get the end of the toothbrush into the clump, twist and remove.
Do not get caught using a significant others toothbrush. Actually fo this I recommend a soft bristle (new) toothbrush. On rocks a scrubby or hard toothbrush may work.
Not all that bad but it can get out of control, some folks intentionally add this as in tank live food.