Yellow plate coral dying

Captain Ron

Reefing newb
A few months ago I purchased a 1" diameter yellow plate coral and put him on the sand bed. For the first few weeks or so he looked fine, but then he started not puffing up much (looked more like colored bone than fleshy coral). I went away for 2 weeks and left the fish auto feeder dropping some pellets every 3 days. After I came back I can see some bone extending from the center where his mouth is and it looks like he may be slowly dying.

(previously my stylophora tipped over and my acan stung and it died 2 weeks later, so I don't know if there may be toxins in the water. I just hung a media bag of carbon in the tank just in case)

Any idea what caused this? Not enough feeding? Too much light, too little light? I have had an algae problem for many months but am running GFO in a reactor and my phosphates are measuring zero, nitrates undetectable. My Ph has always been low. Between 7.8 and 8.0. Have had a hard time controlling alk (I let my alk/calc dosing run dry for 1 to 2 weeks and alk was 7.0 when I checked it. (I have since brought it back up to 10. usually it seems to be about 7.8 or so. I have a hard time maintaining it, probably due to the algae.)

Any suggestions on what I can try?

I would say your algae problems tell me alot of things are off.
The algae is consuming the nitrates and phosphates, that is why they are undetectable.
Any swings in your system have lasting effects on corals. To many swings and they stop growing . Nothing else matters besides consistency. If your alk is at 7, try to keep it there and stop chasing numbers.
Lighting could be an issue, you have not stated what you have. Predators could also be an issue, but with the info you gave I'd go with swing.
+1, what kind of lights do you have?
I have a short tentacle orange plate coral and found that the only way it is happy is in the shade. Any time I try slowly moving into more light, I can see it start to bleach. So it is currently in the shade getting very indirect light and that is where it is happy.
Thanks for the responses. I have a homemade LED light fixture consisting of 24 cree 3watt LEDs, I think 14 blue and 10 white run off a typhoon controller at about 70% mounted 2 to 3 inches off the water (no lenses to focus the light) and my water is 22 inches deep. I wish I had a light meter and a table for PAR values for various corals as I have no idea if my light is not enough or too much. I think you might be on to something Steveg229, I have another coral that has been very unhappy but alive for the past 6 months and I now have him completely in the shade as I think he did not like my lights, even on the sandbed. I can try moving my plate coral under the rocks in the shade as well. I have a pistol shrimp that has moved the plate coral around in the past but I think I can find a place that hopefully the shrimp doesn't mess with the plate coral.

Of course this could all be moot as the coral looks like a brown rock (no tenticle extension or puffiness at all.) I left the lights off today thinking maybe that would help give him a break.

Mariobrother, I can try and see if I can maintain my alk at around 8.5 as that is what I have had it at in the past. As for swings, what amount of swing is too much?