I'm not going to flame you but state from my own experiences, the tank size requirements are there not neccessarily because of the size of the fish but because the fish is an open water swimmer and actually stresses in "confined" quarters.
Does your tang appear to have plenty of swimming room? Probably looks like more than enough, however, the reality is that this fish needs 100+ gallons with a long and low footprint to remain physically and mentally healthy.
As Meowzer stated, your tang is probably stressed and while you can likely nurse the fish through this outbreak, you will likely see the cycle with increasing frequency.
Over the years, having many friends with tangs, the hippos seem to be one of the more easily stressed. I could be wrong, but it's an observation I have made. They seem to be a little less tolerant of water params, tank size, and tankmates, and it manifests in stress.