http://www.seaslugforum.net/factsheet.cfm?base=scutus
"Many fissurellids are sponge feeders, but the general consensus is that Scutus is an algal grazer"
I suppose you could give it a try - it's not like it's a fast mover
(You'd probably have to catch him out at night tho)
What about testing him out with an sps you're not too attached to, put it in his tank to see if he eats it?
edit: there are more threads on them on that page if you check the bottom
hth