Strange creatures in my tank

[DTECH07]

Will they eat your Snails, Crabs, Shrimp, Fish???

 

[Nana Chloropsis]

Bubbles, I would be worried way more about the "transparent feather duster looking things" Have your read up on Hitchhikers (as mentioned) yet? I thought my first aptasia was either a feather duster or a little baby anemone. Hey, that was YEARS ago!

 

[Scampy]

Will they eat your Snails, Crabs, Shrimp, Fish???

I've never seen them harm anything. If i did see bristleworms attacking anything, my first assumption would be that the snail,crab etc. was already dead or dying.
Bubbles, I'll put up a couple links for you to check out on hitchhikers.
Melev's Reef - Visual Identification
LionfishLair.com - Library
Live Rock Hitch Hikers

Hope that helps.