Ok, I found your hermit crab. It's called the hawaiian zebra hermit, and they are great scavengers and algae eaters, but also quite aggressive with any other crabs or shrimp in the tank. If the only things you're keeping in this tank is the 1 clown and the 1 hermit, I don't see where there would be any issues.
With such a coarse substrate, its going to have a very difficult time in finding food. The large claw is used for a number of purposes, such as displaying during mating, protection by blocking the opening of its shell from predators, etc, but it is not used for feeding. You will probably not really see it eat, even if you watch. Being primarily an algae eater and scavenger it relies on dropped food from the other fish to feed... if that food drops into the substrate showing in the picture it will fall between the pebbles where the crab won't have access to it. I would strongly suggest changing over to a sand bed. If you need help on how easiest and safest to do this, let me know and I can talk you through it. It really isn't such a hard thing
As for your clown fish... I asked my husband to take a look at your picture, and he is thinking either hexamita or lymphocystis. If it's hexamita, it will look almost like a small white protrustion sticking out of a hole in the fish's body. Think of toothpaste coming out of a tube. If it's lymphocystis, it will look like cottage cheese stuck to the fish. Lymphocystis is a viral infection and the only cure I'm aware of is the stings of a condylactis anemone. (yes, you could add one to that tank if you replace the substrate with sand). Only the condylactus anemone is known for helping to cure lymphocystis, and most other anemones will be too large for a 12 gallon nano. If it's hexamita, metronidazole can be used to coat the food while carbon is running in the filter. This will allow the clown to eat the medication without treating the whole tank, which would kill the crab and any other inverts in there.
If you're planning to keep saltwater with inverts and fish, it's a good idea to get a 10 gallon tank to use for quarantine in case a fish needs treatment. Medications will wipe out an invert population very very fast, which will also pollute the tank and make the situation worse and more difficult to clean up afterwards.
Here is a link I found with accurate information about your crab: http://www.saltwaterfish.com/site_11...0c5cc3364e7d07
I hope this helps!