It also looks like you have to open a specific app, then use it to scan the code; is that correct? I like that with Apple Pay I can just press my phone's side button two times and then hold it near the reader.

You're right, it's definitely not as easy to launch as Apple Pay since we need to unlock the phone, open the app, and launch the scanner function, but pros and cons I guess. The inconvenience of launching an app is offset by the convenience of scanning a QR code using any phone and the wide availability of the technology.

Do you know anything about how these payment servicers handle fraud? For instance, if someone gets access to my GrabPay account or steals my phone and drains my prepaid account, would I have any chance of getting that money refunded? What about with Maybank where the funds come from my checking account?

As for fraud, I'm not sure how things are handled. I've yet to see any reports of major breaches or even isolated cases, but I'm sure these services have protocols in place for such occurrences.