Though the picture is a bit fuzzy this does appear to be tunicates. They are generally difficult to keep in an aquarium setting so if it was me I enjoy them and show them off! They can be quite beautiful. There were several nice articles in the Jul/Aug issue of CORAL magazine if you can get...
Adult bristle worms generally don't swim. But they do go through a free swimming larval stage so that is a possibility. Regardless it is likely an errant polycheate worm and I wouldn't be too concerned with it. They are generally not a problem in most systems.
Mich,
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