Correct air vent for a gap found in wall
You appear to have a hole in your brick wall that vents to the outside. You would like to put a decorative vent grill over the hole and want to know if there's anything, other than aesthetics, to be concerned about.

This hole will leak conditioned air out of your house. The room will be warmer in the summer and colder in the winter. It will also fluctuate with the outside humidity. Almost always this is not what you want.

This hole may leak conditioned air into your wall cavity. From there it will likely find some other path outside or into an unconditioned space such as your attic.

Either air leak is going to reduce comfort in the room and cost a lot of money in heating or cooling bills unless you live in a particularly ideal climate.

Normally when we vent a conditioned space we use a damper so that we can force air out when desired but not get unwanted air coming in. The spring- or gravity-loaded damper would be part of the vent on the outside wall and the inside wall would have a fan. For examples search on "wall mounted kitchen exhaust fan". You need some ducting between the inside and outside so that the vent doesn't simply feed the wall cavity.

Alternatively, if the purpose of this vent is to bring fresh air in then we would reverse the above arrangement to avoid having conditioned air leave the room.

If you are not in need of a vent then I would suggest plugging up this hole. I suspect that the wood box and rock wool you describe was an attempt to do this. Plug up the hole on the inside and the outside so that you don't leave behind a leak into the wall cavity.

[Editing to echo @isherwood's comment.] Before plugging up this hole I would try to figure out why it's there to begin with. You could ask on SE. We'd need more information on where you live, what else is in the room, what's your HVAC system, before hazarding a guess.