They are harmless organic suspension feeders. They use the tentacles to trap organic particles (particulate matter, uneaten food, detritus etc) and bring it to the mouth for consumption. The tentacles are grooved and they look something like a rain gutter. Smaller particles that drop onto the tentacles are carried down the groove and to the hidden mouth. Larger food particles are lassoed by the tentacles and dragged to the mouth. I have had them for years and never had an issue. They are an important part of keeping your tank clean and help process waste.