these are probably planaria and if you see them then your tank is either being over fed or you arent cleaning it out as often as you should. a good gravel vac usually helps and cutting back on the feeding. they are harmless to your fish too.
That’s the thing there’s no fish in it, I was getting the tank all ready for them. its got silica sand as the substrate, a heater, filter going and a few plants but its been sitting for about a week could this be what it is?