ID help with pics

[atlantis]

With help, I figured out how to post pics. It started off at about 2 inches long then stretched to over a foot. It is purple with no identifyable head or tail but seems to move in a certain direction. Came from under the LR, scoped out the substrate then returned under the rock. Any ideas on what this is?

 

[dogstar]

Cant say for sure....Could be a brisstle or....https://www.saltwaterfish.com/vb/show...ht=ribbon+worm

 

[atlantis]

Thanks for the link. It indeed looks to be a bootlace worm. Evidently they can get pretty big (up to 50 ft) but I still do not know whether to get him out or if he is harmless.