[naesco]

It is probably one of the following in the order listed.

1. Your powder blue tang has a parasite that it is trying to get rid of.
2. Your powder blue tang is not getting enough oxygen (they are very active fish) due to a low oxygen level in your tank or it has developed some gill disease which is causing the tang problems.
3. Your tang is exercising by swimming against the current. Tangs are swimmers. That is why they need large tanks. Ony if the tang is healthy in all respects (clear eyes, active no blemishes, spots or redness on its body and its gills are healthy but your tank is small, consider giving it to someone with a large tank as you have suggested.

The reason why I think is is 1 or 2 is that powder blue tangs in particular are very difficult fish to keep. They often do well for a while and than decline for no apparent reason.

If you have not already done so, position a powerhead about 6-8 inches below the surface of the water. That provides more oxygen and gives the tang swimming exercise.
If this was a naso tang though, IME, it would be normal behaviour.