Sometimes Remove-Zit worked for me, sometimes not. It's perishable. Some collectors have suggested benzoyl peroxide acne paste as a cheaper, more accessible alternative to Remove-Zit.
I'd recommend testing anything on the neck plug first.
I do not store my RC Batman wearing his cowl because of the staining you mention.
WANTED: Dick Grayson SI trousers; gray AJ Mustang horse; vintage RC Batman (Bruce Wayne) head; minty Wolfman tights; mint Black Knight sword; minty Launcelot boots; Lion Rock (pale) Dracula & Mummy heads; Lion Rock Franky squared boots; Wayne Foundation blue furniture; Flash Gordon/Ming (10") unbroken holsters; CHiPs gloved arms; POTA T2 tan body; CTVT/vintage Friar Tuck robes, BBP TZ Burgess Meredith glasses.