Birdsnest losing polyps? Algae on tips? Leave it?

[atruex]

Hey everyone. I got a nice birdsnest colony about a month ago. It has been doing fine, but over the last week or so, I started to notice that the tips were losing polyps, and being replaced with a brownish algae. It almost looks like the polyps are just pealing off. According to my trident, I haven't had any major alk swings. I have my radion xr15 pro set to 70% on the AB+ template. My flow looks to be okay, but I am by no means an SPS pro.

Does anyone have recommendations for what to do/not do?

Edit: forgot to post my params
Alk: 8.31
Calc: 435
MG: 1367
Phos: 0.06
Nitrate: 19.3 (seems a little high?)

 

[T-J]

You can turn your skimmer up to get more nutrients out of the water (skim wetter).
If a coral has algae on it, that part of the coral is dead. Algae won't grow on live coral tissue.
You could snip off the dead ends and see if they heal up.