Can anyone tell me what this worm is? We noticed a couple of dozen of them crawling on the aquarium glass. They've got arrow shaped heads, up to about a centimeter long. I saw one slime across the substrate and on the top they are a dark grey colour. Not seen them before today. Added some amano shrimp and Pygmy suckers to the tank to combat algae about 2 weeks ago. Are they stowaways?? Should I be worried?