It seems like it is getting stressed. Maybe phosphates
Low salinity can and will cause a myriad of issues,being at your levels will be detrimental to probably most of your sps.
It really can be a bunch of things with a new coral acquisition. Could have been on its way out already, it's not unusual for some new frags to lose tissue, it's just the name of the game sometimes. And fwiw having phosphate @0 is not a good thing.
I'd say from the pics the tank is relatively young by all the coral basically being frags so it's really a crap shoot honestly. You gotta get another means to test P04/and alk at a minimum.. API does not have the ability to sustain a SPS tank's levels accurately. Especially the p04 end of it.
But I'll stick to my original comment saying that when it comes to new frags you never really know some may just let go for no reason. If you have issues in the tank is one thing but if you don't and they still shed the tissue it's basically unavoidable Ime with those certain frags. You can attempt to save them but there is those few that no matter what go south. I've had it happen a few times although it's definitely not a normal thing.
I wouldn't focus on that single frag to much, it's gonna happen just make sure your tank is running at its best. Make sure problems aren't arising with your other acros/sps and get some better test kits to see what your levels actually are. Swings are detrimental to sps and not to be taken lightly at any cost.