I assume you have already wiped it with warm soapy water. On my white paintwork windowsill I frequently get the same problem, I use bathroom type cream cleaner (slightly abrasive type) which does not alway completely remove the yellowing, but that fades very quickly in sunlight.
Have you tried sello tape? Put the tape over the area than peal back, if works the pollen should lift up.
This works for Pollen on clothes so it might work for a pollen stain on PVC