You say he's "surface molting" and you can tell because of his behavior - perhaps you mean he's showing signs of pre-molt symptoms?
And if your substrate is gravel, then of course the only way your crab could molt is on the surface
You say your salt water is made from salt you get from sea water your evaporate yourself - can you give us more details about this? I've never heard of anyone doing this before so I don't know what your process might be or the re-mix ratio, etc. Do you use a hygrometer to measure the salinity after re-mixing?
You do need to supplement his diet, drastically. He should be getting proteins, chitin, calcium, etc.
Feed dried copepods, fresh shrimp, and there should be seaweeds as well included.
I'll let Ladybug respond to the rest, as I've got to get back to work.
