Spaghetti worm, harmless and beneficial detritivore. What all has gone into your quarantine tank? It likely came in on something, possibly when very small
Something in the department of "worms that have a lot of tendrils to grab detritus with". Probably spaghetti worm. May very mildly annoy corals if it sets up shop next to them, but generally novelty-to-beneficial. Agreed that it likely hitchhiked in on something while small, or inside a hole.