I have this same alge growing in my refugium only, it started after I had a problem with red slime. I put something in the tank to kill it, and it worked great. My refugium has never grown better since, but now I have this growing, and like you said it does grow fast. I have about four of these little beautys in my refugium now growing daily, just waiting to see what the life span is. I am losing patients with that though, so I think I am going to start removeing it soon. Did it start some where else and move like your sump or something like that???