Unfortunately, Monzo has basically no plans to offer services to businesses, and I doubt they’d want to become a card processor pretty much ever. They’d have to build far too many processes for that, and likely keep it separate from their banking offering anyways.
Asking customers to link their Monzo account to yours so you can debit them also sounds awfully like a direct debit 
; you know you can use GoCardless for that, and the fees are much, much lower, right? 
But on a serious note, that sounds like some form of terms of service violation down the line.