I've seen this type of algae or bloom before with low nutrients, not brown HA if it's what I'm thinking of.. Mine grew in random globs around the tank but looked very similar. I think it's more of a bacterial bloom of some sort.
Manual removal always works best followed by a fresh CUC lots of snails, and bigs ones. Having an algae issue like that with low N&P can really start to starve out SPS.