Here is a pic of this little guy. they mostly hang out on the bottom of the glass. and i seen a bunch on my filter sock. it kinda like like a tiny slug. Thanks. Jason 
Flatworm, not good in large numbers.
gotta reduce feeding a bit and they disappear. i had about 50 and they’re gone. got good advice.
tie a small filter screen with very fine mesh to a hose and syphon as many as you can out of tank. these guys will multiply very rapidly. for every one you see there will be 10 you dont if they are hidin in rock work. i had them bad last year and had to treat tank with chemical to get rid of them. get as many out as possible by syphoning. you can then put water back in tank.
In a very well kept low nutrient high flow tank their numbers don’t often get out of control. In most tanks they likely will. If you don’t dip and QT and repeat the dip 14 days later you will have them in your tank. If you don’t plan to do this in the future do NOT bother to try to remove 100% of them in your tank as you will likely do more harm then good and will just reintroduce them. They are in many hobbyist tanks and in 99% of retail stores and wholesale facilities.
Where they can be dangerous is if you neglect your tank and their numbers get too high. Or even with what seems like a small number during a short power outage and stagnant water(assuming you don’t have battery back up or generator in place). In either case when they die they release toxins which hurt the other animals in the tank. When I got rid of them from my aquariums, not an easy task, the smell from their corpses almost made me gag or pass out.
Keep an eye on their numbers, keep up with maintenance, make sure you have plenty of flow and live and let be. Or do the above and start reading up on quarantine and dipping for flat worms. I use Salifert’s flat worm exit(only product of theirs I use), among other dips/treatments depending on what I am bringing in and what I see on it. Oh and this is one case where you can never trust just one source for your information there is a plethora of misinformation out there on these animals.