I had a similar sps frag do the same a year ago. I just let it be and it recovered, slowly. Lost most of it though. Exact same scenario with my birdsnest.

Crappy... Is there anything else I can do? What caused the rtn? Is it contagious?

I was told to double check my parameters which I did and everything seemed to be in check. Ive never been able to find a definitive cause. Couple things i did around the time of my RTN that could have contributed was changing my light fixture t-5s, beginning to run GFO.

It can be contagious.....if the flesh that is falling off comes in contact with another SPS it can spread.

And as for what caused it....sometimes it just happens for no reason.....but I would say the stress do to being moved to a new tank caused it. Don't fret over it, has happened to me over the years many times.

Should I take it out of the tank?

If you have the equipment to do so cut it just below the white fleshless area. It may stop the rtn

What do I need to use? Can I clip it underwater with clippers? Do I need to stop it from "bleeding"?

I would blame GFO.

Dropping PO4 too fast will cause your SPS to RTN.

Start with 25% of the recommended amount of GFO and work your way up slowly.

Sorry all I meant was a bone cutter

Can I just snap it with pliers? Or garden shears?