Stains around the brush, when using soft brush

Thank you for your kind reply! my monitor is not the best one unfortunately, I work in 8bit mode, I have two monitors and it apears on both of them. 

That's banding. Banding is caused by insufficient bit depth.

If you're working in 16 bits in Photoshop, any banding you see is in your display system, which operates at 8 bit depth (unless you have an expensive 10-bit capable monitor). A defective monitor profile, calibration tables in the graphics card, a bug in the GPU driver, or a defective display panel.

Banding is cumulative, so any of these may add on top of each other.