Damage like you have described on Oled pannels is irreversible. Typically, spots are caused by pannel deamination or liquid ingress, and I'd choose to replace the screen if it is economical to do so.
There really isn't a way to prevent the spot from growing without industal or lab grade tools and expertise. You might be able to slow the growth by avoiding heat and psyhical stress.
I'd inspect your battery there is a chance the spots are caused my pressure from a inflated battery cell.