Accidentally submitted to predatory journal, did not pay fee, already online?
IANAL

It totally depends on your jurisdiction.
However, as pointed out by Solar Mike, they made an "invitatio ad offerendum" (like at the bakery, where they invite you to make an offer (i.e. want to buy) a bread or similar).
By submitting, you declared to make an offer and they accepted it and published it.
So they fulfilled their part of the contract and your part (payment) is still open.

You can still try to retract your paper by sending an email to the editor and maybe it's worth a gamble to not pay. Your institution should be able to help you with this question