Are you sure the polyps were eaten? They can close up, and stay closed up for a good long while - weeks, even.
It's possible that a snail went across them, irritated them and they closed up, leaving a purple 'mat'.
Are you sure the snails are Turbos? There are coral-eating snails - cowries, for example (they don't look like turbos...) they will mow down corals faster than all get-out.
Jenn