It is probably just dirt/dust. It's pretty common to have that settle around the lines/cracks/seams. I often have the same thing happen in my pool. Vacuum very, very slowly and try not to stir it up so it won't go into suspension in the water. That is really all you can do. I can usually do a better job vacuuming if I am in the pool rather than out of the pool. If your pool is still warm enough, try that but be very careful as you are walking around not to stir the stuff up with your feet and move the vac head very slowly.