Welcome to Reef Central!
We as hobbists are on a constant learning curve and with new ideas coming out all the time there is always a lot of outdated information hanging around that is pretty inaccurate.
Just a few years ago most people believed that bristle worms were very bad for a reef and for some reason the idea still persists, especially with clams. The fact is these worms just don't have the jaw structure to eat healthy flesh - corals or clams. They are experts at gumming rotting flesh though. This makes them ideal scavengers and very beneficial to a reef tank. They will take care of left over food, fish poo, dead and rotting critters, etc., things that would otherwise pollute our tanks.
I have five clams in my tank with literally hundreds of bristle worms and have never had a problem. I suspect the reason bristleworms have a bad reputation with clams is because clams often look great right up until they die. Then the reef keeper discovers what he thought was a healthy clam just yesterday is full of bristleworms and assumes they killed it. In fact the worms are just doing what they do best - ridding the tank of rotting flesh.