It's not that "masz" is literally "you have". It's a form of the verb "mieć" (to have) used for 2nd person singular, i.e. singular 'you'.
In Polish, using the personal pronoun is not obligatory and in fact it is usually omitted for 1st and 2nd person, and sometimes also for 3rd. But anyway, "Ty masz" and simple "Masz" mean exactly the same: you have (singular).
"ma" is the form of 3rd person singular: on/ona/ono ma (he/she/it has).
There is a slight but meaningful distinction:
- Mleko z wodą: majority of mixture is milk
- Woda z mlekiem: majority of mixture is water
This is how it is perceived. You can add additional words to describe it better like: mleko z wodą, pół na pół (milk mixed with water, 50%/50%). mleko (zmieszane) z wodą, w proporcji 6:4 (milk mixed with water, the proportion id 6:4).
This rule is not strict. For me "płatki z mlekiem" = "mleko z płatkami", milk with flakes. Probably it can be used for items of same or similar type.