Most hobbyists encounter a strange calcified tube with a web coming out of it, which is usually noticed during tank feedings. The web is cast by a Vermetid, a snail that uses the web to capture particulate-sized foods in the water column. Using its jaws, it consumes the web with the food attached. While it is common to have a few, Hop discovered that his tank was the perfect breeding ground for thousands of these creatures, which led to the demise of his reef. The webs irritated nearby corals, often shutting them down entirely. You can break the tubes off carefully, or plug their holes with Super Glue Gel or putty as a way to thwart their existence in your tank. Personally, I've never seen more than a handful in my aquariums, but you may discover some in the plumbing, sump, or affixed to submersed pumps.
Photo by Hop.
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Here's another image of the pink or reddish tube of one in my 400g reef. Photo by melev.
You can see the web it has cast to the right of the tube, used to capture a meal. The coral to the right isn't faring well due to the worm's efforts.
You can make out the jaws of this worm here. The larger circle at the bottom of the opening is the worm's protective door that it uses to seal itself in from anything that might attack it.
I have seen vermetids in all sizes, from 1/8" wide tubes to tubes thicker than your thumb. In the photo above, you can see how the coral is growing over the hard tube's walls.