I wouldn't worry too much. Get a plastic mattress protector if you are concerned. Pretty much anything can be cleaned off a mattress. You can use orange power carpet & upholstery cleaner on mattresses and it works a treat even for pet stains and odours.

If I ever get to have an advanced pregnancy my biggest fear about waters breaking would be having the same thing happen as did to an old friend of mine many years ago.

Her waters broke while she was waiting for her fish and chips to cook at the fish and chip shop. She was mortified!!!! She just stood there, the guy behind the counter asked her if she was alright, she said yes, collected her takeaway and went home and ate it before heading off to hospital (it was not her first birth).