Planaria

From The Aquarium Wiki

Planaria are a type of tiny turbellarian flatworm, found in tropical and coldwater aquaria. They generally live in the substrate, feeding on dead and decaying organic materials. Although they are not harmful to fish their presence in high numbers suggests that you are overfeeding and under-maintaining your tank. A water change and vacuum of the substrate usually gets rid of these. Some fish like Gourami or Pleco may also eat them.

Monitor your ammonia or nitrite levels in the aquarium as these flatworms tend to make an appearance on the sides of tanks as they attempt to escape the toxic water.