That's normal. Not sure what they call it, but my tang's white line comes out at night along with him getting pale and a brown spot around the white line. It is a defensive measure I was told.
My tang only does this at night. If yours is doing it during the day, it may be stressed out. If so, what else do you have in the tank? Has your tank matured (i.e.how old is your tank and has it finished cycling)?
HTH.