If she doesn't mind drinking soy, you can let her finish it. I used to do that.
Now I divide the soy powder into 2 portions. Upon opening the tin of soy powder, I'll scoop out half & keep in the fridge. I also mark the expiry date on the container.
My thought will usually consume about half of a small tin within a week or so. She used to get loose stools a few times per year when she was btw 12-24 mths. Nothing serious except she can't take milk.
If the container wasn't air-tight (ie no metal lid was used), FM tends to harden & you can see small clumps in about a 1 month or so. I'm not sure if you can still re-fridgerate yr tin cos it has been opened for some time.