1.) You are correct, a tang of any type shouldn't be put in a tank that size.
2.) If you do indeed have the dreaded BRYOPSIS, a tang will not touch it. One of the only fish that will eat it is called a Foxface Lo, which I have in my tank. He eats it, but mine is beyond what one fish can handle. Incedentally, BRYOPSIS is poisonous to most fish, tangs included.
Tell me, does your algae sway in the current like evil, green prairie grass? Mine seems to have a couple different stages of growth. First, it starts off as a slimy, short fuzzy-ness to the rocks, then it grows longer and stronger to sway in the current.
Edit: spelling