Plaster will fade from sunlight, which is normal.. This one looks like chemical problems.. Hard to tell from here, but low or high PH can cause major problems. Low PH will eat away at the plaster, which could have caused the pitting and uneven surface. It also looks like there is some scale, which would be from high TDS and/or High PH. Dark colored plaster can be very difficult to take care of. As a pool builder and designer, I try to steer people away from dark plaster because of this. I prefer a darker pebble style finish instead.