I used Grab in Singapore before. It's the same app, no different versions.

But there is a separate e-wallet for SG. You can't use the balance in SG e-wallet in Malaysia.
Also, if you use your credit/debit card to top up the balance in SG, you can't withdraw it, or transfer to other people.

I still have balance in my SG e-wallet, can only use it in my next trip
And, QR code payment isn't very common in SG