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Had them in my tanks for ages but I can't tell you exactly what they are. Totally harmless detritus feeders. You may see more if your rock has collected pockets of detritus. I always took them as a sign that it was time to clean my live rock with a small power head before my next few water changes. They aren't bad one bit.
HOB filters generate their flow at what is directly in front of them, which is the back of the rocks. One method of removing debris from the front of the rocks is to put the filter on the front of the tank for a day or two. You'll be surprised at how much detritus it can pick up like this. You could also use a turkey baster to suspend some of it in the water and let the filter remove it that way. What size tank are we talking about?