Agreed, looks like a scutus. To know for sure it will occasionally expose it's small thin whitish shell, which will look like a stripe down it's back. Althou if calcium is low in the tank it may loose its shell.
Far as I know may it eat zoas and other soft stuff, but also eats algae. When small, if you see no damage, it prolly does more good than bad. When large, and they can get pretty large, the damage will prolly increase, and they are more able to bulldoze stuff around. See no problem with keeping them. They don't multiply quick and become a problem, and if you have a strain that gets large, simply take em out when they get large.