Same issue and have tried "Pure Clean" cycle several times with no effect. We're really tired of picking these mold flecks off our clean clothes and some have even stained the clothes in spots.
Our model number is WA456DRHDWR/AA
It sounds like the unit is not able to drain properly. I would recommend inspecting the drain hose first. Check the hose •Is not inserted more than 6 inches into the standpipe/sink and is not kinked, bent, or damaged. Also, do you have a drain hose extension kit on the hose?