If you have a cantilever deck (bullnose) instead of coping, you should have a control joint around the beam of the pool. Typically that's ~12" from the inside edge of the deck's tip. If the builder didn't put one in there, you'll see cracks exactly like what you are experiencing.

You can use a RotoZip type tool with a diamond wheel. Smooth each side of the crack til you are satisfied. Fill the crack with dry sand within 3/4"-1/2" of the top of the deck.

You can then use a self leveling seam sealant (Home Depot sells SikaFlex) to fill the joint. Be careful, it has the consistency of syrup and is very difficult to clean.

We use a commercial grade product called BOSS by Steigmeier, not sure if it's available in retail.

Good luck!