USD and Riels are interchangeable. They are very careful about the quality of the US bills they accept. No writing or rips allowed. And small denominations are best.

This doesn't answer your Grab question but may help if you decide to go down the cash path.

Once in town we took Tuk Tuks almost exclusively and they are cash. We only took a cab to go to the Killing Fields. (Well worth it by the way.)

I tried Passapp. In my experience it was cheaper than Grab but with less availability. YMMV.