What is “mei chalav” exactly and why is there a de’ah in Shulchan Aruch that it should be ossur to consume?
“Mei chalav” is the “water” residue when cheese.
The Torah permits milk, although it derives from a live animal; it likewise permits cheese. The mei chalav however is not milk nor cheese, and therefore some prohibit it. The broad consensus of authorities, however, and certainly for Ashkenazim, is that it is permitted.