is this something they do, or something I need to worry about?

they were completely retracted yesterday, and I did a 5 gallon water change last night.

Temp: 79.10
PH. 8.2
Alk: 3.5 MEQ/L
AMonia:0
Nitrate:0
Nitrite: 0
SG: 1.023
Calcium: 445ppm

I have seen this guy in a retracted state before, but never this long, and never this completely. The xenia, colt, and rhodactus are happy as can be.

I suspect this might be the problem (if there is one) the tank just had one of those 'new tank' algae outbreaks that has been pretty well controlled by the cleanup crew, but I do think there might be a little actually growing on the coral... should i clean this off with a q-tip or something? it is not hair, that I can tell, It is a brown film. I have read starpolyps can be overgrown by hair algae, but this doesn't even cover 50 % of what it's encrusted so far.