How bad is this huge monster :-(

The only thing "bad" about this "monster" is what you're doing to it!

 

I have always seen bristle worms as beneficial, they eat dead things and keep the larger porous holes in your Life rock flowing freely, the only thing I would fore see as bad is if your Aquarium is infested with them if something goes south with the Chemistry of your aquarium and there is a die off they can Poison things, but if your like me and have Corals you will see the adverse condition effect them way before the Worms are effect, I would think with fish too but?

Fire worms are about the same only they can sting stuff Like You, Fish, Corals.
fish will usually stay away from them if they are smart, exception being the Puffer fish they think they are delicious

 

That little guy...I wouldn't worry about that little guy.

 

Bristle worms are friends not foes

I have a few and thank them for keeping my sand and rocks clean

 

me too I like them

 

I find them very useful, never had one go rouge.

 

FWIW, the Hawaiian Fireworm, which is one out of hundreds of species of bristleworms, is the carnivorous predator. The rest tend to remain beneficial members of one's CuC.

 

I have plenty of that. I usually saw them when I lift up any corals from sand. Their bristle stung my fingers, every single time! Its not dangerous just painful and swollen for a couple of days.

 

they creep me out