Ropy or slimy milk happens when certain bacteria produce a thick, gum like polysaccharide on the surface of the milk. Alcaligenes viscolactis is the organism most commonly responsible for this, and because it needs oxygen to grow, the sliminess shows up specifically at the surface rather than throughout the milk. Streptococcus lactis is instead linked with the normal souring of milk, and E. coli is better known for stormy fermentation. So surface ropiness points to Alcaligenes viscolactis.