My first guess would be a heater burn. You have the temp a little high in that tank (per my opinion) unless you are working on a special grooming for him (which I don't encourage s it can shorten their lives because grooming them is mostly stressing them into a certain look). I would start by turning the heat back to 79-80 range and keep up with the clean water as it sounds like you are doing and watch him. If you begin to see any signs that the spot is getting bigger or beginning to look fuzzy you know its not a burn and likely a fungal infection instead. In that case you just need to get some fungal guard and treat for 5 days with water changes. But watch him first and lower the temp so the heater isn't so hot.
Noticed discoloration a day ago. Woke up to this. Really nervous. Don't know what to do have been searching threads all morning. Water parameters are amonia 0 nitrite 0 nitrates 10. Ph 7.5 water temp 84. He has been with me in these conditions for 4 months. 
