I woke up this morning to find that my male betta had a large grey/white mark on the side of his body which has developed around the rest of his body as the day has gone on. I have no idea what this is!
Someone please help me and Dave (the fish).
My first betta died in the same fashion as yours. I didn't get to see it happen but when I woke up one morning he turned to a true black from a dark blue color. I suspected it was because he didn't go up for air very often and probably drowned, but I can't be certain.
Either way, I don't believe that it is an illness but a full water change will remove any ammonia that was expelled from his death.
If he was the only fish in your tank I would wash/scrub it with a bit of bleach (about 1/2 teaspoon?). Rinse till the bleach smell is gone and let it air out for at least 24 hours. Don't know what your setup is.