You have most likely come out ahead because the no show fee is typically the whole stay's cash price, which can be an obscene cost depending on the room, time of year etc. So "cash fee" should be read as "alternative cash price for the stay". The cost of a points no show can be especially painful because people often book rooms they wouldn't book for cash! People have lost thousands and thousands of dollars with this mistake. The hotel has, probably, done you a favour by checking you in, as they've guaranteed you'll only be charged points.

That said, if you have mistakenly booked a stay, you might be able to get the hotel to cancel it after the first night. If the hotel has marked you as checked in, it sounds like they're aware of the no show points trap[1], and they are trying to be helpful, so they are probably willing to cancel the stay after the first night for you, which should in theory get you most of the points back (or perhaps that is what they are already doing and you will find out tomorrow).

So, next step is to speak to the hotel, let them know that you're happy for the first night to be charged (in points) but you would like for the rest of the stay to be cancelled, because it was a mistaken booking. You might even get a full refund, if they are especially kind.

[1] A hotel gets paid by Marriott for points redeemed at the hotel but it is at a much lower rate than cash, so a hotel stands to gain financially from a points booking no show, and loses out by checking in a points booking. I guess in theory the hotel might know it's a nightmare to get no shows to actually pay up and some points is better than no cash, but it is also possible they are doing you a kindness. Hard to know without their confirmation.