New vinyl liners often have a build-up of a sticky substance that can be coloured or clear. Sometimes it sticks to the bottoms of feet and turns black. This is plasticiser migrating from within the liner, and is usually only prevalent in colder water temperatures with new vinyl liners. Brushing and shocking can help reduce this build-up, as can increasing the water temperature. This build-up usually goes away on its own as the liner ages and the water temperature warms up (about two to three weeks).
On other surfaces other than vinyl liners, a gooey substance can build up on the walls and bottom. This is usually a build-up of body oils and other insoluble compounds and can be eliminated with more frequent shocking and brushing.