How many fish (though this shouldn't be a factor since you are skimming, and your water tests fine)? How often and how much for water changes? How long is your light cycle? Under what type of lighting?
The Xenia frag
I picked up from the LFS
had bubble green algae and green hair algae -- I removed both manually with my fingers, popping and plucking all of the bubbles, and hand ripping off the hair algae -- all tossed into the garbage. Neither have returned or grown anywhere else in the tank. Of course, I run my UV
My light cycle is probably way too long ~8:30am to 10-11pm (depending when I feel like discontinuing observing and going to sleep