Is this STN on my Montipora?

[FishguyJosh]

Over the last two months I lost most of my more delicate SPS like acropora which I attribute to some uncontrolled alk swings..this problem I have rectified. And now it looks like I have some slow tissue necrosis on a good colony of montipora spongodes? The dying tissue seems to be working itself up from the base Even though at the top everything looks healthy and it's still growing.

Should I assume this is slow tissue necrosis? And should I try to frag the healthy sections or wait a bit longer?

Kh 8-9
Ca 440
Mg 1320
NO3 15-20
PO4 0.05
SG 1.025

 

[Dburr1014]

That could be shading as well

 

[FishguyJosh]

That could be shading as well

Yeah I was hoping it was shading (because it really doesn't get any direct light ) but the area without colored tissue seems to be moving upwards

 

[Dburr1014]

Maybe need some more opinions.
Definatly watch it. If it keeps climbing you will have to frag it to save it.

Vermetid Snails??

 

[UMALUM]

At first glance I was thinking shading as well but with the continuous recession it could be anything. I've always dipped in iodine and went the super glue route before completely chopping it up.