Whenever Tuya Bearclaw attacks, it gets +X/+X until end of turn, where X is the greatest power among other creatures you control.
Once the ability resolves and the bonus is applied, that bonus won't change, even if the greatest power among other creatures you control does.
The value of X is calculated as Tuya Bearclaw's ability resolves. If you control no other creatures at that time or if the greatest power among other creatures you control is 0 or less, X will be 0.