Damn, sorry for your loss. Did you remove the body? Did the firefish just die all of a sudden too or was that earlier?
The cloudy water is probably from the fish dying. In a new tank where the bacterial community isn't well established, something dying can spike your parameters and cause a bacterial bloom, which makes the water look cloudy. I'm guessing that the spots were in fact some sort of disease/parasite.
What to do in the meantime is first test your water. I suspect that either ammonia or nitrite will be present. Then you'll want to do some hefty water changes, like 10 to 20% every day or every other day until your water clears up and your parameters return to zero. Then you will want to leave the tank empty for a few weeks before adding any new fish. After that, stock very very slowly to allow your bacterial bioload to catch up to the fish in your tank.
Do you still have some fish living in it now? If so, then the water changes are even more important.
Don't get frustrated, stuff like this happens to us all. Setbacks are unfortunately a common part of this hobby, but you will get through it.
