Ahh yea, I see Mike!
Joker, it will probably continue to grow back until you deplete whatever it is feeding off of, probably from the sand, you may want to do as said increase the flow down there some & practice vacuuming lightly over the sand surface during wc's & right now, I'd do a few super large wc's back to back to help knock it down some.
Believe it or not, LARGE wc's will help you fight most algae just because your keeping a glass box of water that isn't constantly replaced, every so often you need to jump on those large changes to straighten things up.