Zoa Issue

[Denver45]

Hey all, so I noticed a zoa in one of my groups shrinking this morning and then just found another one. I haven’t noticed any issues with them before. I did start to change out the sand bed on Monday, but did a very small section and have been monitoring my parameters very closely. Today’s Parameters are as follows:
Salinity-1.024
PH 8.0
Ammonia-zero, possible trace less than .25ppm
Nitrite-0
Nitrate-2.5
Ca-460
Phosphate-0
KH-8

Picture is below. Other zoas in the same tank look fine so far. Any help would be greatly appreciated.

 

[najer]

Have you checked for pests?

 

[Denver45]

I did check and haven’t noticed any as of yet.