The little bit I know about Red Sea Salt is that it is a slightly different combination of ingredients from most "marine" salts, because the Red Sea is slightly different in trace composition from the Indian, Pacific, Atlantic, etc. Alot of people raise fish from the Red Sea, and so using that salt is preferably for them. This is what I understand at least.
It will be fine to use it though for your crabs. I wouldn't worry about it. The biggest worry about salts at this time is the long term consequences of using Instant Ocean. All other salts are better! lol
I have been using Oceanic recently as well, and I like it! I save the Doc Wellfish to put in their food sometimes. Oceanic disolves so well and so quick!
Gertie