It's not digesting per say, but attacking it.
As for anything being safe by acans, not really. They don't have a long range, but will absolutely dominate anything that gets close. I can a colony of zoa's fall on an orange crush. It couldn't have been there for long (couple hours at max). The zoas were almost completely wiped out, with the acan polyps just slightly less puffy. Within 15 minutes the acan was back to normal and you'd never have guessed anything happened.