I’m sorry to disrupt the beauty of this line with a sort of schoolyard thought. . . but milky head definitely seems like it could be an awesome insult. Here, it’s meant to suggest a white-haired old man, maybe with a shade of kindness and compassion and nurturance given milk’s association with motherhood and the milk of human kindness and so on.
But if some kid shouted at me from across the merry go round that I was a milky head, I might be upset. I wouldn’t know what he meant exactly but I’d think he was alluding to a kind of thickness of mind, a clogging of synapses, a sloshing quality of thought. In the cartoon version, I turn my head and milk spills out of my ears.