I'm suspecting that this crack is causing squeaky noises when the region holds weight. What's the best way to seal this?
Well it would help to have more information, This looks like cement backer that was cut. not a crack created by normal movement. If it is a crack RUN OUT OF THE COUNTRY! Because cracks don't show up 1/8" wide and and are gone in this short distance. The noise problem could be caused by the structure., not being glued and screwed or just cheap construction. I am not sure it really needs to be filled and I am sure it is not the root cause of the squeak. If you or any one out there can develop a "crack in plywood " anything like this you would make the record books. But if it is just old vinyl glue that makes it look like backer the problem may be more screws are needed, maybe some shims from the bottom. But filling the cut won't help.
That is a layer of a product called homosote, at least it is called that back east. Your subfloor is under that. Homosote is not structural, it is usually 1/2" thick and resembles gray cardboard.
To get to the squeak, you will need to set a circular saw a 1/2" deep and cut out a section to get to the subfloor below, say maybe a 3' square. The joints are every 4' apart in one direction. perhaps there is a factory edge that is near to the squeak that simplifies the section you need to remove.
After you get that up and out of the way address the squeak.
With that much homosote out of the way, you should be able to see 2 rows of joists. You may only have a squeak at one location and perhaps the cutout can be smaller than I suggest but I would screw down the neighboring joists as well while you are at it.
Since there is a subfloor under the homosote it would not take that much effort to pull all the nails out of the whole sheet instead of cutting out a section.
If you do choose to cut out a small section, PLEASE do confirm the thickness of the homosote.