Single piston calipers have a slider mechanism, but this is a twin piston and so doesn't need one as the two pistons counteract each other.
It looks like the pads have worn down quite a lot, which has allowed the piston to rock slightly as it comes to the end of its travel (as there is not much piston left in the bore it will do this). The force of the braking effort on the pad has caused the piston to rock, digging the front of the pad into the disc more.
Its mainly because the pads are worn so far down. new pads will fix this as the piston won't be able to rock in the bore. Cleaning all the crud off & exercising the piston in and out a couple of times (push it all the way in with the old pads on & then put the caliper back over the disc & press on the brake pedal... repeat a couple of times) will help.
Not a major problem but it could be if the pads wear down so far that the piston falls out of its bore !!! Change the pads before they get this low.