I have a similar problem with an additional complication: The crack fills with dirt that appears to be extruding into the pool from behind the tiles. I can clean out the dirt but appears within a few days. Leads me to think there could be some worm or bug burrowing but I have never seen one. I plan to use a hose to jet the crack, allow to dry and then caulk with silicone.
There are various brands of 100% silicone caulk at Lowes/Home Depot: GE, DAP, DuPont, 3M. Does anyone have a recommendation as to brand?