I have never heard of a crab getting ich! I didn't think inverts could get it, but never really looked into the question.
You CANNOT use copper on inverts. Very bad things happen
Remove your ich-ridden fish to a Q tank--probably the best plan for you since it sounds like you don't have a tricky reef setup-- leave the crab alone or check out
www.wetwebmedia.com for more info on inverts and disease treatment. AGAIN, do not treat your display tank with copper. The above website has advice on setting up and using quarantine and hospital tanks... Pretty easy to do...it just takes a while to get rid of the disease. Pay attention to the kind of copper you get...there are two kinds and the copper tests are type-specific.
Also, a good thing to do is look up the marine ich lifecycle. It'll give you an idea of why you have to treat your fish for so long.
The Tetra food won't hurt the crab, though--if that's the way you're going to go. That particular food is pretty hard to find, though. Tetra no longer produces the anti-parasite food. Check around your local fish stores, though. Lots still have some on the shelves (at least they do around here). If the only kind you can find is the "stick" form (flakes are tougher to come by), grind some up in a coffee grinder and feed somewhat sparingly for a couple of weeks. But if you do not have a UV sterilizer to kill the free-swimming parasite, then you'll have to do daily water changes and gravel cleanings to help control the free-swimmers. Which is not fun
You can also check into raising your temps and playing with your salinity to help control the ich parasite while it's free swimming. That I've never done, though. So again, info can be found at wetwebmedia.com.
There are a lot of ways to treat ich. The sure fire and easiest thing is copper. Copper copper copper. You'll get so you hate the word!
Best of luck...!