If these are indeed nematodes (roundworms), they are probably getting nourished due to overfeeding. Reduce feeding (even go a full day without) and keep that regimen up for awhile. They should starve without any other help. In the meantime, scoop out as much as you can since they are collected in that spot. A few PWC
's within the same week (ie: If you are doing 30% weekly, do 15% twice a week) will also help.