My user name is a genus of seaslug. I love seaslugs! Scutus are slug-like (often mistaken as a slug.) And I scan the threads looking for titles that might be slug related, in the hope that I can learn more about them or help someone with the little bit that I know about them.
Just like the Asterina starfish, people go back and forth on the Scutus spp. Calfo's and Fenner's Reef invertebrates book claims that they can't be trusted, however wetwebmedia (a Calfo Fenner beast) says that they can be trusted. I trust the seaslugforum more, and their reports say that the Scutus is a herbivore. I have one that is quite large; I believe 3" long.
I'm looking for more because I'm all for animals that are attractive and eat algae. They also are of a specific gender, so if I want them to reproduce in my tank, I'm going to need a few. And people only ever get them as hitchhikers from their live rock. Trying to get another one that way is going to be very hit or miss!
I definately wouldn't flush these guys, especially if you haven't seen them doing anything bad. If you can, post some pictures so we can be sure, but your description seems spot-on.