I have two in my 125. Both turned all black (or is dark blue?). I had one that died after 5 years, so these guys are relatively new (about a year).
They each have their favorite rock holes to dart in and out of, but also dart from rock to rock. They let the big fish grab the nori first, them come for their nibbles. When I had only 1 it was probably the most entertaining fish to watch. Two more than doubles the fun.
The two occasionly squabble, but that lasts about 30 seconds before one them stops chasing the other.
If you have a 30 gallon or larger I highly reccomend this fish.