LPS Skeleton Dissolving

[falconut]

Here's a photo of one of my Candy Cane Corals. The bottom right has two old skeletons were the heads had died a long time ago. But, the skeleton is starting to dissolve. What's harder to see is at the top, just to the right, the tiny head is hanging, because the skeleton is dissolving under it.
http://i1031.photobucket.com/albums/y374/falconut/Reef%20Tank/mini-DSC_2353.jpg

When I did research about this it mentioned that skeletons can dissolve if the ph is too low, but that doesn't appear to be my case. I have listed my parameters below:

Salinity: 1.026
ph: 8.4
NO3: 0.2
PO4: 0.00
Alk: 9.5 dkh
Cal: 410

My 90 gallon display tank has an estimated 100 gallon total. It's been up and running for almost 7 years. I run (2) 250 Watt MH's for about 6 hours a day, with (2) 110 Watt VHOs on only 1 hours before and after. I am running 500 ml of EcoBAK pellets, which have been running for over 1-1/2 years. I also use GFO in a reactor. My skimmer is a ASM G-2. My refuge has about 7 mangroves, sand and LR rubble.

I believe this is a chemical breakdown, so maybe somebody on here can give me some help on figuring this one out.

[tmz]

No it wouldn't be the ph or your rock would dissolve too. That threshold is below 7.7ph usually in any case.
I've seen caulastea do that. I don't know why. Something to do with the coral's calcification process.I suspect. Caulastrea and a few other fleshy corals, don't do great in systems dosed with organic carbon particularly polymers or monomers ,ime. They may not adjust well to very low PO4 and NO3 or maybe the nature of the change to the organics mix in the aquarium or by products from bacterial activity have a peculiar effect on them .

[falconut]

I'm not completely sure my system is totally low in nutrients. I've been getting some cyano on the sand and a few isolated spots of HA, but very little. So, there must be some.

[bertoni]

I agree that it's not the pH. I'm not sure what's happening, either. I suspect the coral is altering its skeleton.