Why? It could've just been someone with fucked wiper jets trying desperately to get some fluid on their windscreen. Unless they were actually squeezing a bottle of fluid on you... kinky.
I've seen people in motors doing it to other people on bikes, and it's happened to me before...and it's a shitty thing to do at best. At worst it's assaulting someone with stagnant and bacteria ridden gloop.
Report them