For those who had acquired bali aquacultures from lps in the lower mainland, I found a number of these crabs in them. Their common name is Gorilla crabs.
They have the distinct black tips on their claws and the hairy legs. I thought they were friendly acro crabs until I found that they were moving from acro to acro munching away at the tissues on the inside hard to reach areas of the acro. There will be no polyp extensions in the area that its "hanging" out inside the acro and you will start noticing fading colors in the area as the tissue is being removed.
Getting rid of them is quite easy though with a skewer or those small metal skewers for tying up a chicken. The crabs are quite brazen in that they will seek refuge within the acro and not run away - they will clamp themselves down with their legs/claws when you put the skewer towards them. THEN just stab them through their shell and throw them into the toilet. Do not try to use a tweezers unless you are confident in your ability to grab their bodies on the first couple of attacks. I tried tweezers on one and I ended up pulling off 3 legs before it decided to run away.
They are also quite easy to spot because they are quite active within the acro.
I am confident they came from the bali's because my tank was crab free with my interceptor treatments and they were the most recent new purchases with crabs.
Keep your acros happy and kill these suckers; in return, your acros will love you.