If it is a light red to gray color with white clusters of thin thorns/bristles on the side - oh yes, definitely bristleworm or fireworm. Harmful? That is an ongoing debate and since there are over a 1000 specieis, there is not one absolute. Some argue they aerate the sand and are beneficial. In my breeding tanks, I know they will eat nudibranch eggs so if I see even one bristleworm that has ridden in on something and sneaked past QT, I break down the tanks and restart with new sand and rocks. In my regular tanks, I leave them unless they start appearing in every nook and cranny - or if they get so big they look like they could eat a clownfish (I have run across two that looked that big).
You might want to search the Internet and look for bristleworms or fireworms to see what they look like and the impact they can have on your tank. Usually a large population of them indicates high nutrients in the water.
Warning - if you try and grab the larger ones with tweezers, they will usually break apart and you will end up with two. Don't touch them with your hands - take it from my experience, those bristles break off in your skin and hurt!