Bryopsis is a pain in the rear. I had it. Suggest taking the rock out if there is nothing in it like corals or anemones. Scrub with brush and spray good grade peroxide on it, dunk into ro/di, then let it sit in clean premixed salt water for 3 days covered with no light and a little water movement. Before putting it back in, give it a scrub and spray of peroxide again, rinse in ro/di.
Yes you will probably kill some of the beneficial bacteria, but trust me. Better the bacteria than a full blown bryopsis take over.