Reef safe "ish" I would say. The problem is they're big and and have pointy sharp things all over them, and bump into things. Fish are usually smart enough not to swim into a needle but when I had a diadema in my 115g cube I'd see the occasional broken off needle related injury (once a tang, once my golden dwarf moray - both times the fish healed up in a few days). And they'll knock over any frags that aren't secured down somehow.
Having said that, I did find the diadema ate valonia (bubble algae).
I have another urchin .. I wish I could tell you the latin name but I can't find it. It was sold to me as a "green urchin". Looks pretty much exactly like your typical tuxedo urchin except that it's green. Stays small (think raquetball size), eat algae including caulerpa and valonia .. although has the annoying tendency of all tuxedo urchins of picking up and wearing whatever it can (reef rubble, frags, nori elastics, etc.). I've tried googling for info but can only find info on a coldwater species (that looks like it, but it isn't, because I've had 2 of these now for about 5 years in my reef, if what I had was coldwater it surely would have kicked off by now).
This would work better in a 50g IMO. If you can find one, go for that instead. They look like this:
... failing that maybe try a smaller diadema and see how it goes. You may need to rehome him as he gets bigger (it doesn't take long - I bought my diadema's at around the size of a quarter, wee little tiny things, but they were fully grown within a few months). But you can always try trading him into your LFS for a smaller one maybe.