Concrete dust drips :(

[Inspector14]

ok, i have a new '09 Meteor metal LB, i live in an apartment tower that had a parking garage built 2 years ago. i was assigned a space on the lower floor (its alaska, and the car stays clear of snow in winter).

The problem that is occurring is: when it rains, or when snow melts in spring, or when someone washes their car, basically whenever water is put onto the upper deck of the garage, it drips trough the cracks and joints in the concrete.
the water then pools in the metal sub-floor of the upper deck and drips through the ceiling of the lower deck.

this water is full of concrete dust/particles. and it leaves concrete on my car! big gray splat marks and runs, i can pick the dust off with my finger, but if i hit it with a wash sponge- goodbye clear coat and paint. luckily this stuff doesn't seem to bond with the car body it just sits on the surface.

is there a best way to take this crap off without really damaging the car? like using a clay bar it on them?

thanks for suggestions.

(i have also considered getting it professionally detailed with professional products, and sending the bill to the owner of the apartment tower. i am not the first this has happened to. one guy had just bought a new Chevy 2500 crew cab truck, beautiful bright blue paint, owned it 2 days. it rained. his truck was literally, mostly gray from concrete. ruined his paint.)