I would say it was left in too bright of a window sphag turns white and yellowish when it gets sunburned and i think ur plant has been badly burned.. Too much direct sunlight in high heat is sometimes worse then you'd think its baldness to me when one of the kids i kno left a plant on a sunny table for a few hours...it didn't survive. Im still holding on to it to see if it offsets but idk if it will. Goodluck