That is a cold air return so yes it is fine. If you want to hide the vent, paint a piece of wood, put spacers on the back then apply it over the vent. It won't hinder the vents function at all and will camouflage it a bit.

There is no sheetmetal ductwork in the wall. They simply use the cavity space between the drywall as the air shaft. The good thing, you can see exactly where the 1 middle stud is that you have to mount to. The bad thing, you can see in your vent and you'll be staring right under it at the tv, not like walking by in a hallway. What I have done is remove the grille and paint the inside of the entirety a deep dark brown. You can also flip the vent so the louvers are facing up.

What 3onthetree says!

"They simply use the cavity space between the drywall as the air shaft."

You're saying that based on what specific knowledge?

Cat Simonson, whether or not there's a duct there can be determined a variety of ways. There's a device called a Walabot that shows what's behind drywall using radio frequencies. A metal duct may deflect a magnetic stud finder. Looking under the wall (basement or crawl space) and above it (attic) would show ductwork if there is some.

I believe using a cavity for a return air passage is no longer permitted by code, it needs to be ducted. While existing ones don't need to be changed, doing so would improve your HVAC performance and reduce the dust blown around.

Pop off the cover and look inside.

The vents in my home are ducted.

@Elmer J Fudd did you look at the photo? Pretty easy, you see the studs and the grille crossing over 2 stud cavities. A ducted return would probably be oval within the wall, with a boot or a plenum box at the grille. Neither present here. But it's all moot; the tv mount must go into a stud. There is no concern about a duct in front of this stud. No expensive equipment and exploratory surgery needed. Overthought and overdone.

So again, they simply use the cavity space between the drywall as the air shaft.

No, and I wouldn't try to assess something so important from a small photo. Others are welcome to do so.

Using a wall cavity for return space is an inferior approach to save costs and is as bad as having a ducted passage with holes in it. It's uncommon in my area and if I came upon that in my house, I'd change it.

@Elmer J Fudd, I certainly hope you wouldn't gut your house for a tv mount, but that's your prerogative. And you wouldn't be able to connect the new oval to the branch above or below it without some major access to get the work done. And unless there are open gaps under the drywall (usually an HVAC tech will put an 'L' piece of sheetmetal at the floor during rough-in of the return cavities), there is no new dust or debris, beyond the sawdust that was sitting within the cavity from cutting the hole in the bottom plate/subfloor or top plate, to be sucked into the return duct (which the cfm is not strong enough to lift the sawdust anyway). Whether it is not the best method to use open cavities has no bearing on this OP trying to get their tv mounted. And to throw you a bone, if there were a duct in a wall, and a lag bolt went through it instead of the stud, it would still not affect the return or allow extra "dust" into the return - at least until the tv falls off the wall and pulls out the lag bolt. And we, yes you included, use pictures that OP's provide to communicate their issue that we advise on all the time. If the OP is trying to diffuse a bomb, a picture might not be adequate, for a tv mount the one provided will do.

I didn't realize you were one to argue moot points like some others do so often.

Sorry, I'm not sure what you thought was argumentative. I didn't see what you're saying you saw in the small image. If you did, great.

I don't consider air compartments in walls to be sealed and airtight - far from it. If you do and are fine with the negative pressure pulling in who knows what from the open passageways, again, great. It's something I would remediate or might even have avoided buying to begin with..

Use the studs to mount the TV then no issue as for the vent it self I never get why a cold air vent is in the upper wall when cold air goes to the floor not the ceiling. Maybe that can be moved why not get an HVAC gut to check it out. BTW you also need to make sure you mount the TV at the proper height so the center of the screen at eye level when you are seated is the ideal. maybe post a pic of your whole space too.