Baal was one of the ancient Canaanite gods, one of those deities worshipped as king of the world, dying and resurrecting every year, also was god of storms, fertility and cattle.

His name got somehow twisted into an evil god because of ancient Hebrews who smeared him incessantly in the Bible since it was also one of the main gods of hellenistic Seleucid Empire and Selucids were forcing Jews to woship him, causing tons of religious tensions - and they were doing it in the time that significant parts of Bible got compiled so naturally the current issues took main place in those.

Moloch is also Canaanite god, but it's kinda discussable if it was even a separate god or just other title of the same god (Moloch is just Melek which means "king", while Baal means "lord"). Also got smeared in Bible and later christian theology for the sacrifice of newborns. And while that practice is confirmed, it's still unclear if it's even tied to Moloch/Baal and most probably the sacrifices werent alive but the babies that were stillborn or died soon after birth, since newborn mortality in those time was huge.

In short, it's incredibly convoluted part of ancient mythology with tons of missing context, but the Elder Scrolls daedric prince Molag Bal is a combination of both names in fully biblical sense since as the god of rape, cruelty and domination he's probably the most evil and nasty of all daedras (which is an achievement since the competition in evil and nastiness is harsh there).

It's likely the other way around. Molag Bal - Moloch Baal. "Baal" just means "lord" or "master"