I had the same problem lately with an Acro. It looked like RTN but started at the tips, not the base. Mine only lasted a few hours like this, it started late one night, and the coral was gone by mid day the next day. No other SPS in the tank was affected. For now I'm assuming it was too close to my lights...it had been in my tank for about 3 weeks, and I didnt let it acclimate to my lights.