What is it when you pee and there is this heavier substance in the toilet water that moves like oil ?

Parasites don't swim in urine like that .. most likely this is either some mucus or tissue coming out with the period or it's something wrong in the urine itself, some times protein in urine ( if you are diabetic or have some kidney disease ) can cause the urine to have something thick in it .. go get a urine analysis ( not a culture, analysis would tell in less than 24 hours )