that looks nasty! i'd try to get as much of it off as possible by hand, then i would start with warm water and some mild "meister proper"

. if that doesn't work, try a product like decal remover or petroleum ether. you should be able to get most of it off eventually ... of course, you need a lot of patience. don't rush it, rather take your time, because once it's ruined, it IS ruined for good.