What are the brown spots on the bathroom wall?

i steam up my shower real good and i think it may be my tub is dirty, the dirt mixes with the water and floats in the mist, the dirt condenses in spots on the wall. but i wanna know fo shao because its strange to have a bunch of brown dots appear all over the walls. the shower steams up enough to the point where the walls are drippin, so i think i might be right on this one. the spots seem grainy in nature and rub off quite easily. I have two cacti (w00t) in my bathroom, sometimes mah kitty knocks em down and dirt film covers everything, and i dont ever completely get it all in the cracks or corners this might be what causing it. what do you guys think? dirt? mold? fundots?