What type and how many fish are in your tank? Were any recently added? Were they quarantined? If not, a new addition could have brought a problem in with it (all fish in the stores are not healthy, even if the guy says so). From what I've read it sounds like some sort of fungal infection, or possibly something foreign (air freshener, windex, etc) may have gotten in your tank by accident. If your bioload is too high they could be stressed out. A yellow tang alone, gets too big for a 55. If its a fungal infection, you need to quarantine them, in a separate tank (some medicines have copper that will kill your inverts) and treat outside of your main tank (copper in the medicine will get in your rock,sand,etc) and stay there. You could also be getting a bad mineral( copper, phosphates,which feeds unwanted algae hair,cyano) if you are using tap water. I only use RO/DI.
