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 tube of one in my 400g reef. Photo by melev.