There is individual variation in all species, but most likely it will leave corals completely alone when it gets a case of the munchies. Other encrusting animals (various worms, Vermetid snails, bivalves) will be on the menu and animals that are easily damaged by being sat on/bumped/etc. will probably not enjoy the hermit's company. Basically, standard large bumbling animal precautions apply:
- Stick your rocks down or make sure you have to invest a fair bit of effort to move them, don't stack them precariously
- Don't put fragile things where they can be fallen on
- Don't put fragile things where they're likely to be between a strong animal and something of interest (trapped food, etc.)
The main possible problem I see isn't so much the reef safeness as that you've stocked it with smaller hermits. If the others are a different species, you will need to keep a close eye on the lot over time. Calcinus elegans is usually pretty peaceful as Calcinus species go, but mixed-species environments (particularly those with size differences) can trigger many unwanted behaviors that are far easier to avoid in single-species environments.
