Acropora, lossing tissue in multiple spots no clear culprits

[Joshua Sassaman]

Ive had a large colony of this green acropora colony, its losing tissue ina few spots, I have a few ideals, but still no answer. Here is the tissue lost and I just recently took it out to examine for many possible parasites. Ok only this came off. Im not 100% sure the Id of it. RTN and STN were my original guesses but its not acting like then at all. I Have ideals but want some freah eyes to see it with no bais. The worst part is all other colonya ars normal. Also this colony, polyps are always extended and is growing faster then all my colonys.
PARAMETERS
Ca: 450
Alk: 12
Ma: 1500
Po4: >1
NO4:>1
Salinity:1.022
Temp:79
Pump: 1 Mini Polario 10ML
Tank size: 56 column
This fell of:

The colony and white spot, this happened after a water change and is found in some corners of where braches meet

 

[Flippers4pups]

Dip the acro in coral dip and see else comes off. The larger picture look like a seroild.

https://www.lionfishlair.com/hitchhikers-guide/

 

[Joshua Sassaman]

Dip the acro in coral dip and see else comes off. The larger picture look like a seroild.

https://www.lionfishlair.com/hitchhikers-guide/

Seroild? What is that?

 

[Flippers4pups]

Seroild? What is that?

Type of snail.

 

[Joshua Sassaman]

I cant seem to find that snail, so is that a pest snail? Or a problem, is another name by chance sundial snail?, also I will dip it tonight or tomorrow

 

[Maritimer]

That might be because "serolid" is a family of isopods. "Sand skaters" are one variety. Sand skaters, IIRC, are nice to have around, but I can't promise that about every serolid.

~Bruce