I do agree with the "banding" result, but in my experience is it not caused by compression(*) but by heaving post processing:

• If you start with a JPEG image, the dark parts of your images are coded with very few bits, so there aren't very many possible color combinations, which isn't much of a problem because our eyes don't see much color in dark areas. If you do some color analysis on your picture, it contains 13K colors... but only 180 in the background, with only about 50 colors in any 200x200 area.
• When you lighten these parts, you don't add more color combinations but the color themselves, being less dark, are more identifiable as colors, and the 1-bit differences between dark colors have become much wider value jumps. So where the value jumps in contiguous pixels groups you see a hard limit.

Away to avoid this is to "dither" the colors before processing. If the 8-bit picture is loaded as a 16-bit (or more) image, pixel values can be fuzzed a bit to take advantage of the full range of values (a bit as if you added random decimals to integers). This suppresses areas where all pixels have the same values and so value jumps occur at more random places and become invisible.

(*) heavy compression can cause it but it also comes with other artifacts that aren't visible in the submitted picture.