This sounds like a fire worm (hermodice carunculata) they are redish brown and have white legs with fring above each leg. Bristle worms are in the class of Polychata and come in many colors, sizes, shapes. Though there are many, they are all considered aquarium pests. Some genera can reach over 20 inches in leangth.
They reproduce sexually and a sexually. They are both carnivores or omnivores. Either way it needs to go. Your worm could catch a fish at night when it is sleeping. They produce very quickly, especially if the population of thier species is down.
You need to get him completely, as if you were to get part of him the remaining half that is burrowed in the rock will form a new end and quickly reproduce.
Please act quickly, because bristle worms are worse than hair algae. In that they will actually consume everything in your aquarium.
I take a pvc pipe about 1 1/2 inches cap the ends and drill a holes big enough for the worm to fit into. Baiting it with muscle, squid, shrimp. insid the capped pvc. They will eat the bait and not fit out of the whole, kind of like catching a monkey. They are preditory creatues and will go for the bait at night. In the morning take the pvc and empty it in a hole outside. Be careful they sting and bite depending on the species, yours especially thus the name. I use a couple of these traps at night on my reef every few months.
Good luck. I'd get a picture, 'cause in about 5 years your going to love to look at it.
