It could be someone got something on it, or it could be the PVC was improperly manufactured and is bleeding out plasticizers. That's a known issue with PVC and if that's what you have, all you can do is try to clean it and prepare to replace it. If you are the original owner, you might want to contact Zodiac
and see if they will make any accommodation, although I expect they'll just say "no longer under warranty". You never know, because this would be a "latent defect" and they might still be [reluctantly] obligated to do something.