its artifacting. often cause by bad drivers. bad oc or excessive temps...
steps to cure it... reinstall the drivers, if that doesnt work try the 12.2 performance drivers (i know they work perfectly and have updates that 12.3 doesnt have).
if that doesnt work. make sure your running at stock speeds for that card. ie turn down any factory oc that came with the card.
then if you still have issues then check the temps
if your gpu or vrm temps are over 100'c you need better cooling. turn up the fan with either amd overdrive or msi afterburner.
also you may want to get better fans for your case (make sure your fans are pulling front to back side to top... in other words front and side fans pull air in. while back and top pull air out of the case... this gives the best airflow for cooling the innards...
This isn't the only thread about this problem lately. Seems that since CCC 12.3, Radeons have had this problem (that includes my 6870's). I haven't had time yet but I'd imagine rolling back to 12.2 would help.