Grey/Black Sand?

Hey guys,
I've had my reef tank for about 3-4 months now and during the 3 month mark, I had a big tank crash which was caused by the unnoticed death of my mandarin goby. When i found the goby, only skeleton was left. The tank water turned yellowish brown and I did 50% water change which did not help. Later, all my fish died($140+ worth). I also did numerous tests and did not discover anything odd. However after a month of waiting and constant water changes, my water turned back to normal. Recently, I put in a damsel, just to make sure and it's been swimming fine. Yesterday, two of my snails died, and some parts of my sand on the right side of my tank turned greyish black. I wonder if this has anything to do with the death of the goby and if any bacteria caused from the death is still present. Lastly, the first dead turbo snail I took out had none of its meat left in the shell. I am scared that the snail's carcass will again foul my tank. Please help. I am also really worried about the grey sand so please tell me if you guys know what it is. I can provide pictures of the sand if you need them. Thank you! Happy Thanksgiving if you celebrate!