snails pest

Hello I found these little critters roaming my rock work. Are they a pest or beneficial to my reef?

About a month ago I found two of these on a snail they would go from being right up on his shell to being stretched out about 6 or 7 in really thin they almost look like they had little tentacles or feathers at the end of them that are hard to see in the pictures but I hope you can get an idea...