Help lots a critters have popped up within a day

[wyatt2847]

All these things have literally popped up within 2 days and i have no clue what they are looks like some kinda bristle worms and a sea slug maybe some hydra and some other weird mite looking bugs how would i know if the bristle worms are dangerous to my corals and fish

 

[wyatt2847]

Some. Better pics

 

[WheatToast]

Looks like a flatworm.
https://reefs.com/magazine/pest-free-reefkeeping-iii-the-flatworms/