That looks like a coral and not any sort of algae. Are they growing fast? It looks like some sort of clove polyp to me, maybe brown clove polyps? Also look at pictures of brown star polyp which look similar as well.
If it’s confined to only one rock I would remove that rock personally to keep them from taking over. If it is an invasive clove polyp you are going to want to deal with them sooner rather than later. They can take over a tank.
I think fenbendazole might kill clove polyps but I’d sooner take that rock out than treat the whole tank. It can possibly hurt some inverts, including leathers, stars, and shrimp. Hopefully someone who has experience with fenbendazole will chime in.