To me they look like tufts of hair algae. If you are able, pick it off with your fingers to manually remove. If you can't do that, try a toothbrush to scrub it off.
Ideally you would keep lights off while bacterial film grows on your rocks, about 2 more months. If you've already added photosynthetic creatures, consider reducing your lighting schedule and adding a cleanup crew that includes blue leg hermit crabs while your tank equalizes over the next few months.