8dexgqAn, (1) we use el halib when we want to make it definite (معرفة), ie. ال doesn't always mean "the" in English but we know which halib is exactly, and (2) we use halib when it is indefinite (نكرة), ie. we don't know which halib is exactly.
Just "this milk". Without the definite article, it means "This is milk". (Remember that Arabic doesn't use a verb in the present for "is"; the definite article is often used to clarify when you're using a noun predictively like in "This is milk" or attributively like in "This milk".)