I can understand the dark staining from copper. I suspect your copper levels in the water have increased over time and reacted with the TCCA (,trichloroisocyanuric acid). But I don't know why a sticky/greasy substance would be coming up from the drain. Do you know what your pool plumbing is made of? Is it all PVC material? Any other chemicals added to the pool water besides TCCA, or acid perhaps?