flatworms, some can reproduce and take over your tank. they live off of detritus and excess nutrients. Sometimes they can use up all the excess and just die off. I had some in my old 65 gallon tank and could not for the life of me seem to get rid of them. Some six line wrasses eat them so thats a potential resolution to your problem.
if there red your in trouble if there clear or whiteish clear your ok those are hunters the red ones bother your corals.If you seen the timelaspse video i seen of the red ones youd be shocked .Plus the red ones multipoly real nasty.The white ones stay in check and are nocturnal