Most of you know what the difference between milk and cream is, right? When milk is left un-homogenized, a thicker, fattier portion raises to the top. That stuff is bottled seperately and used as a substitute for milk. It’s not as healthy, but it’s generally stronger than regular whole milk.
I imagine that Aurthour’s lusus milk is even thicker than that. Not just the “foamy head”, but all of the milk. All of it. It’s like drinking a thick milkshake almost.
Though that’s directly contradicted by the fact that it can “quickly evaporate” after Equius spills it (which would mean it’s probably thinner than milk). But still, it’s Homestuck, things don’t always follow logic.