The 43-year-old former 'Spice Girl' flirted with Efron, 30, via a dating App.
This led them to exchanging flirty texts before they actually hooked-up.
And shortly after she split from Belafonte the two hot stars enjoyed a night of passion at her Los Angeles home.
But despite their chemistry, it's reported that the stars were happy with just a one-night fling.
The insider added: "It wasn't exactly a meeting of minds. Neither saw it as long-term, but there was chemistry there. Mel thought Zac had an amazing body."
Meanwhile, The Blast website is reporting that Mel B requires a 'responsible adult' to be around when her 7-year-old daughter Madison stays the night with her as part of her ongoing custody battle with Belafonte.
The court order says Mel B can "Hire an acceptable nanny from a registered and bonded agency" to satisfy the requirement.
However, there must be someone "present" so that the star is not left alone with her child unsupervised.
The performer was also ordered to undergo testing for drugs and alcohol in random tests.
The court will be allow her to suspend testing during periods of travel, but then must "test within 24 hours of her return."
Mel B is also allowed prescription medication that has been submitted to the court.
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