Piston seal is gone, the rubber is just dustboot.
Easy to change, just get the part and some brake fluid.
Bleeding it is a bit hard as I had to drive it for a day after initial bleeding, then bleed it again before it became firm.
If you dont do anything to it, you will lose your clutch (like I did while coming to traffic lights, but mine failed rather quick instead of slow leak) as master cylinder starts taking air from the tank.
Also remember to put little grease in the cup where the piston head goes.