No. Do not dip rocks in fresh water - too many beneficial creatures will die and this will upset the balance of your tank.
To see what you have in your tank that you do not know about you have to wait until about one hour after all the lights have gone off in the Aquarium and room where the Aquarium is situated.
The get a red light (or some torch with the lense covered in a red transparent) and check out what's about in your tank. You may notice a crab, mantis or a nasty worm (not all bristleworms are bad - most are very good scavangers that clear dead and decaying matter).
If you do see an unwanted worm, you can either get it out using one of these traps (I've tried it and it works).
http://www.marinedepot.com/aquarium_miscellaneous_coralife_trap_em.asp?CartId=
or with tweazers - if you are quick enough to grab it (do not touch them with your barehands as the bristles are an irritant and some can be very painful)
For crabs/Mantis you can get a Mantis trap that should catch them.
http://www.drsfostersmith.com/produ...&Ntx=mode+matchallpartial&Np=1&pc=1&N=0&Nty=1
Or even build your own trap. Do a search on Mantis trap and you will get a lot of detailed info on this site.
Otherwise see from what rock they came from and QT that rock until you catch it.
The worms that you see with spaghetti type tentacels across the sand are good worms and should be left alone.
After you catch your suspect. Take a pic and post for ID in the invertabrates section and you will get a reply if it is a good or bad critter.
If it is bad (and nobody wants it)- the best way to dispose of it is by freezing it.