It sounds like you're just seeing the rubber of the outer tread skin flexing open. Provided there's no cut threads at the base of the cut the only downside to that tear or cut is that it may tend to pick up stones easier and it'll let water wick into the thread casing more easily.

I believe if you do a search you'll find some threads in this mechanics forum about gluing such tears together using CA "superglue" or similar.

It's not uncommon for my tires to have multiple small tears or cuts where shards of this or that have cut the tire. And I certainly don't bother repairng puncture holes in my tires. But I admit that if I had a cut that was much longer than about 3/16 to 1/4 inch long in the rubber tread layer I'd try some options for glueing it back shut. But only if it doesn't have a lot of cut through fibers at the base of the cut.