GSP is insanely hardy even if it closes up for a bit it’ll probably be fine. When I first started my tank and I got GSP I also had some algae on it. I originally tried to turkey baster blow it off but some remained. I used a basic extra soft toothbrush to scrub it and it was completely fine in a few days. The GSP might also still open just fine with some algae on it like it does for me if you can’t get it all off but have some CUC who will pick at it. So you could try a turkey baster but the brush should be fine. I’ve used soft brand new makeup brushes on soft corals too when my tank was younger.