You should give him all the seaweed he can eat (i.e., there should be some in the tank all the time). Frozen lettuce (romaine not iceberg) works. Tangs like to graze all the time and if your tank has no algae in it.....Algae is not really that nutritious so they need a lot of it. Probably rubbed his nose raw trying to eat something in a crack.
Also, I don't think tangs have scales.