PCB Solder Mask textures
The pattern is on the copper layer. It appears they have used the "hatched" pour option in the PCB design software with an unusually low fill. You can see the regular pattern of the pour being broken up by tracks and one area where the pattern is missing completely, presumably because the "remove dead copper" option was used when making the pour.

It looks different from normal because the solder-mask is an opaque black rather than the normal translucent colors. So while on a normal PCB you see a color difference between copper/non-copper areas here you see only the texture.