It is a BTA.
Judging off the color of your rock your tank is fairly new/young?..
IMPE nems have a much better survival rate the larger they are (4"+) along with being introduced to a more mature and stabile tank with sufficient lighting & flow.
I've always heard and stuck with a minimum of 6 months to a year of tank maturity before adding a nem...
Is that written law, can you be successful earlier?.. I'm sure it can and has been done, but waiting that time frame has never let me down.
Obviously nems will move to where they want to be, but they do not handle quick parameter changes well, especially seen in newer tanks with newer reefers.
It isn't uncommon for nems to hide a week or so after being introduced. Keep an eye out for excessive mouth gaping, color loss, shriveling and "throwing up". Those are usually an indication it is on the way out.
My buddy has a BTA that he battled with for months. Parameters where bang on, but it never looked "healthy". He simply made the switch from Chinese LED's to quality T5 's and it is now out and better than ever today.
I successfully keep different types of nems under AI LED lighting. They all gape from time to time and expel waste from feeding. They will shrink and expand throughout the day as well.
Clowns host most of them and every BTA I keep has lost its "bubbles" for some reason and from what I understand it is unknown as to why, but they are all healthy so it doesn't bother me.
Just to give us some info; how long has the tank been up and running and what are the parameters?