welcome to TRT! They are a risk in a reef tank. They arent listed specifically as eating crabs or snails that Ive seen or read, but some of the recomended diet is fresh water clams and muscles. They will nip at coral also. It can depend on the individual fish. Hopefully someone will chime in who has one. I dont think its an issue with them though.
no they will not eat crustaceans or snails. there mouth parts are to small for that. they will however eat tube worms, and if you are lucky, maybe aiptasia. the have also been know to bick on button polyps and other small polyped corals.
I had a copperband in my reef for about 2 years then I sold him - he ate all types of worms and aiptasia but when I got a bulb anemone he started picking at it. Other than that, he was a good scavenger to have, never bothered my crabs or shrimp.
They pick at random corals and clams. I just lost one 2 days ago(had it about two months) . I'm positive it slowly starved to death, even though I have over 200 lbs of very nice LR for it to pick at . It just never took to any kind of food I offered. Make sure you see it eat BEFORE you buy it, as this is one of the most common problems with these fish.
The big question is will they EAT. before you buy, you should make sure it will eat frozen Mysis. Mysis is about all they eat, other than aptasia. Make the LFS feed the copperbanded BEFORE you buy, and watch closly to see it it eats. They have a poor survivor rate in home tanks. Picky eaters and poor collection practices make them iffy