You know, that's funny I was just thinking that it sure looks like Dictoyota, but it's totally the wrong color. A Diadema might do the trick, but they grow REALLY fast! Like really, really fast.
Take a hunt through here maybe. I already have, and can't come up with anything, but you'd be better at recognising it since you look at it everyday.
http://saltcorner.com/sections/guest.../algaepage.htm