Red Planaria.
I have written extensively on treating them, but here is the short. Levamisole is a dry powder that treats worms in livestock - this is the same stuff that is in Salifert Flatworm Exit - 1 tsp in a 500ml water bottle and then start at 1ml per gallon of tank water. The treatment/med is quite safe, but the toxins that the flatworms put out is very dangerous. Need to suck out as many as you can, have tons of water for water changes and half a dozen reactors full of high quality GAC. They can crash a tank. Also, need to do three treatments, but the second and third ones are nothing and just get a few stragglers.
If you want to know more, then hollar. They are somewhat harmless until there are so many that they cover everything. They are eating film bacteria and algae, diatoms and dinos on the rocks and you can get blooms once you kill them all.