Zoanthids dying?

[RookieDiver]

One of zoanthids frag seems to be dying. I am new on this and kind of lost on what I should do. I had it for 2 months no issues, but last week started to look weird and it is getting worst by day. the only change I did was adding new corals, they seems to be doing fine. Please advice!!
Thanks

 

[2021 reefer]

Are your parameters stable? What are they?

 

[RookieDiver]

ALK 165
pH 8.1
PHOS 0.4
Ca 408
Mg 1372
Ammo 0
Nitrite 0
Nitrate 25
Salinity 1025