mine is doing fine with about 2 watts per gallon in a 125 reef. it randomly closes and opens when it feels like it though. if there are other corals around it they could be bothering it
that is a fungal infection from the sounds of it, yellow tangs stay yellow, you should quarantine him and treat him ASAP before it spreads to the other fish
mushrooms, ricordea, kenya tree, pulsing exenia, colt coral, leather coral, are often very friendly. with sps corals and lps corals, just give them enough room and they will be fine