I grow grape caulerpa and chaeto in my sump. Whenever I throw a handful of either in my display tank it's gone by the next day. My yellow tang, foxface rabbitfish and sailfin take care of that. I recently also got a vlimangii tang too. I think he wouldn't let that stuff stay long either. I don't know what a powder blue would eat. My point is that if you put a small amount in there, your yellow tang probably wouldn't let it spread. Calcerous algaes like halameda are another story. My tangs won't eat that. If it does spread, I think you should be able to pull it out pretty easily.

Jack