If it is important to you, replace it. If it’s not important, remove it. Or just live with it. Not much you can do to fix it.

Mortar isn't glue, as the person who slapped the mortar in that joint found out. He made the problem worse by providing a larger lever for the freeze/thaw cycle to use to separate the walls. If the wall is otherwise sound, I'd ignore it.

Another vote to ignore.

Something like this of appropriate height would help:

My bigger concern is the rest of the wall. If it were mine, I'd replace it or at least have somebody evaluate the wall to see if it is sound. Is the footing adequate? How was it filled? How much if any rebar was used? My concern is based on living where there is seismic activity. I have a similar wall around my property.