First issue, is although the sandbed is best loaction, sand can get on it and irritate it or stress it to the point of tissue recession as you have.
I place mine on a rock to elevate it a little. They also like moderate light and water flow. The light seems a bit bright.
Feeding is another requirement 2-3X per week
Then come water:
salinity 1.025
ph 8.1 - 8.3
alk 8-9
mag 1300
CA 440
Nitrate < .05
Phos < .04
Temp 77-79
It may recover as non tissue area is white. Once it turns color- its done. If so, You Can superglue the dead are and the remaining should remain healthy