The only way DE can get into the pool is a tear in the grid. You might have to replace the grid.
I have midnight blue, but in mini-pebble. I also think it's lighter than I originally thought it would be based on pics. I love it but wish it was just a shade darker. And of course, the lighting makes a huge difference in how dark the water looks. I had some mottling and some is still there but it doesn't really bother me, some mottling is to be expected with pebble, not sure about Quartzscape tho.