Big dave, you really need to do some basic research. He has a 40 with NO fluorescents , not much live rock and a big fish load. Could be diatoms or slime algae - if you syphon it all out, how long does it take to regrow.
You don't have enough light for the coral, and to be frank I'd imagine you have pretty horrible water quality, thus the tang will be totally prone to any infections.
My honest suggestion is to go to Clearwater aquatics (lok in phone book ) or similar, and get some reading material. Your current system is not likely to be successful in my honest opinion