amoled display assemblies, like the one you have in your phone, are made by fusing together the amoled screen itself and a protective gorilla glass panel. They use a special UV reactant glue to secure the bond in such a way that when looked from above the glass cover remains transparent and the glue layer is practically invisible.
What happened in your photo is ..well, kind of unfortunate. For some reasons, the glue between the glass and amoled panels got damaged/unglued. It doesn't feel to the hand since it's actually underneath that surface of the gorilla glass panel. Unfortunately, i don't know any user DIY way to solve this problem. For what i did encounter with amoled screens, this type of damage mostly occures when exposing the display to high temperatures. This - coupled with some manufacture defects in the targeted area, results in this kind of problems.
Normally in order to solve this problem, the phone would need to be sent for repairs and it would either be exposed to UV light to try to reglue the panels or have the gorilla glass removed, replaced and the glue reapplied.
Please take note that some repair centers don't have this type of UV equipment and may suggest a display module replacement altogether. That's very expensive, about the price of a second hand phone.
Xperia Z Ultra
Surface Pro 2
Doing Phone/Laptop hardware modding for some years. Seen a lot of broken things in my life.