I really don't have much to add to Kevin's post. I'm sorry I didn't see this before.
That is definitely RTN
- not bleaching.
you should remove the acro and frag as many branches away from the necrotic area as possible. Placing them in various places in the tank with varying lighting and flow, with any luck one or two frags will survive.
This is the best advice that I could give. Personally, I would also do an dip of each piece in Tech-D -- >>here is some info. on that product<<
-- if you can get your hands on some quickly, that might be helpful.
Do you have other acros in the tank? Keep a close eye, RTN
can spready rapidly.
Also, meticulously check every possible tank parameter. If *anything* looks off at all, do a water change. I'd also try and run some fresh GAC
if you can.
Not sure what to do about the crabs - hopefully they'll relocate on their own.
Good luck, and keep us posted!