Theory: symbiotic relationship. mermaids produce excessive milk at all life stages so the starfish drink it making the starfish well feed and the mermaid milked (which is important in over production of milk)
I always figured mermaids would be more like cetaceans than fish. Having scales on their tails would be inaccurate, but breathing air, growing hair, and having a horizontal tail fluke are all in-line with mammals/cetaceans.
Not to mention, it would mean mermaids would have regular genitals (albeit hidden inside a genital fold, to increase hydrodynamics.) It would also mean they’d have normal placental pregnancies.