Phoenix, you need to get on top of it now! You'll probably end up losing the rock that the xenias are on if you really want to keep it from spreading.
I ended up throwing out about 50 lbs of rock. It made it easier to remove the remaining rock pieces for scrubbing also.
I lost several corals due to the stress on the tank. I did 2 50% water changes within a month and a half. And then 20% water changes monthly.
Every time I'd see a patch, on rock, or tubing or sides of tank, stuff got moved or taken out, cleaned off and put back in. Sometimes this was everyday for a few weeks!! It was definitely a pain. But it has paid off.
I had MH's on my tank, I went back to my PC's. I may try the MH's again in another month or so.
I disassembled and cleaned the filters every couple of weeks.
I also took the temp down to 74.5, I use to keep it up around 79.
My parameters were always good. Nitrates, nitrites, PH, alk, calcium, salinity etc, etc.
But start now. And keeping the rock with xenias will be touch, because you'll have a very hard time getting all the hair algae off without killing the xenias. You may decide the best thing to do is dump the xenia rock.