Wikipedia:
Crop milk is a secretion from the lining of the crop of parent birds that is regurgitated to young birds. It is found among all pigeons and doves where it is referred to as pigeon milk. An analog to crop milk is also secreted from the esophagus of flamingos and the male emperor penguin.
In other words, certain birds regurgitate this substance in their throats called 'crop milk' (or 'pigeon milk' if you're strictly talking about pigeons) that they feed to their young. Though for some people who don't know what that is, they may think that Brewster is talking about...
Another type of 'milk'...
So that means Brewster thinks of the player as his child and throws up crop milk into your coffee.