The necrotic tissue is in the shade which I wouldn't worry about. Acros are a little sensitive and can slough off tissue without warning. It's not bleaching, this occurs at the tops not the shaded ares. It has good growth around the base and seem healthy in general. RTN is fast! If it was RTNing it would probably be dead by now. The acros that RTNed on me doubled each day in dead tissue. Most of the acros in my tank have had a small dead patch appear on them overnight only to regenerate soon after. Its a fine piece though, where did you get it?