Palys dying off

So over the last couple weeks, I've watched about 2/3 of my palys go from a healthy brown to a deep green and die off one by one. I have roughly 5 healthy polyps left, with the rest all fading.

I haven't changed anything about the tank in almost 6 weeks in terms of water quality. I have done weekly 15% changes and check levels every week. Nitrate is a bit high because I'm a day or two behind this week's water change. Tank is about 3 months old, and up until this point, the palys have done great. They were hitchhikers and have tripled in number since I got them.

pH: 8.3
temp: 78
Ammonia: 0
Nitrite: 0
Nitrate: 10
phosphate: 0.25
calcium: 420

I added an anthelia a couple weeks back, and the only thing I can think is that it is putting a little bit of shade on them, but nothing I'd think would affect them like this. They still get plenty of light. The anthelia, my torch, and my duncan are doing just fine. Thriving even. I have 2 frags of zoas, which are also doing perfectly fine.

I added a six-line about a week and a half ago, but I have yet to see him touch the palys. The rest of the characters have been in there for quite a while and there were no issues.

Anyone have any idea what might be going on here?