So an angel can use any angel's grace? What's with that?

It is like a form of magic that she used to help people with.

He isn't using it he is stopping Anna from using it. :)

I think we will see some negative effects of using another angel's grace as this season goes on.As for Anna why would she want to steal another angel's grace, probably murdering them in the process, if she didn't have to? And it certainly wouldn't be an easy thing to do.

Anna wanted to regain her own grace to stop the other angels from killing her. If she'd killed another angel or turned an angel human in order to gain grace, the angels would just have another reason to want to kill her.

An angel each has their own grace that is a part of them. An angel will not have powers without grace but they can take another angel's grace however they will not be at full power. Also if you take another angel's grace and use it it deteriorates slowly over time which would cause them to take another angel's grace once it fades completely. When angels die their grace is the blinding light that is seen when they are stabbed.