can i keep sea apple and hermit crab?

I wouldn't know myself, but what kind of hermit crab is it? Someone who knows the answer will likely need to know.

Many hermit crabs collected from the shoreline will require land to get out of the water on. If you can't provide this, it shouldn't have been added to the tank. Since it has, see if your LFS will take it--animals put into an aquarium should never be released into the wild, as they may introduce diseases.

On the sea apple--they can poison the tank. What do you have in with it, and are you target feeding?

Most sea apples are filter feeders. Get some invert food, and target it with a baster when it's feeding tentacles are out. Very few places will accept inverts back--it's a risk for them, since they don't know what's in your tank. The fish are unlikely to pick on the apple, but watch the hermits. They shouldn't intentionally mess with it, but might annoy it by walking on it or pinching a tentacle.

And, you really might want to do research before buying/acquiring anything else. It's much easier to prevent problems than to resolve them. Even with hardy fish, you're likely going to have problems simply because damsels are very territorial and will beat the heck out of each other. Clowns are in the damsel family, and while they don't pick as large an area to defend, are just as nasty about defending it.