Well Spot is named spot for a good reason.
Anyway if it has been several days with just the one spot (other than his normal spot) I doubt you have ich. I do not however have any idea what it may be.
Get a qt tank going in case it is something that needs to be treated with meds. It is a good idea anyway so might as well get it over with now since it will need to cycle before you can do anything with it.
Try to get a pic if you can. Even if it is blurry th experts may be able to figure it out. Ich usually looks like the fish was sprinkled with salt not just one spot.
HTH