The skimmer wouldn't have that large of an effect on an lps for that short of a time....photo? Might be brown jelly disease or maybe just not healthy when you got it....hoot will help. #reefsqad
Good chance he was injured if you're seeing signs of an infection....not good.
It'll be hard to say when it happened for sure, but shipping and handling can be pretty rough on them!
Did you take any measures to match his current light conditions to where you got him? Taking some lux or PAR readings will do the trick and avoid any added stress from too much or too little light.
Any nitrates in your water?
If your water is too clean that can be a source of stress.
Your alkalinity is slightly low....is it stable at 7.3? How would you characterize stability of your tank's alkalinity?