What are these?

I am just being a worried parent but what are these and are they a threat to the tank?



Below the bubble coral







I removed this next one from my tank. Looks like a centipede in the water.







I have heard of bristle worms and stuff but i have never seen them. Can you guys tell me if this is stuff i need to remove from tank or let it go. I removed the worm looking guy but the coral/anemone looking thing i left.