It is probably something that I found an article on - “The stains are caused by a strain of bacterium known as Streptoverticillium reticulum that infests boat upholstery between the top layer of foam and underside of the vinyl,” It usually happens to fresh water boats, it can't live in salt water so you don't see it much there. From that article -
“This bacterium needs four things to establish a colony in vinyl upholstery,” Laster explains. “It requires oxygen, fresh water, warmth and food.” The first three elements are easy to find in a lot of boats, especially ones in humid climates. Food, on the other hand, requires people — the bacteria feed on the oils and sloughed-off cells from human skin, as well as the chemical para-aminobenzoic acid (PABA) found in some sunscreens.
Check sunscreen labels and packaging, including the fine print, for PABA (para-aminobenzoic acid). PABA is linked to pinking stains on boat upholstery.Courtesy Panama Jack
Pinking occurs when the bacteria run out of food and begin to die. “Through a metabolic process, the bacteria secrete a red dye,” Laster reveals. “The dye is then wicked up through vinyl material, and this results in blotchy pink or light red stains on the surface of the upholstery.”
Preventive Steps
"To help prevent pinking, you need to eliminate the elements necessary for the bacteria to get established," says Laster. To cut the food supply, clean the upholstery thoroughly after each trip. If you're not washing the boat, wipe down all the vinyl with a clean towel at the end of the day. Also, avoid the use of sunscreens containing PABA.