It might be the increase of light. Possibly cut down an hour or so, but gradually (if you have corals). Still seems like there might be another problem as a huge hair algae break isn't usually just from an increase of light. It might be from your source water
. And a foxface will grow up to 8" (min. tank is 70 gallon). There is controversy about large fish like blue tangs going into small tanks too, but IMO
the foxface is too large for that aquarium, even if it's a young fish. There's other fish that eat hair algae like the lawnmower blenny, turbo snails, and seahares. It's good that you're trying to find the source of the problem though.