Since we moved into our house several years ago we keep finding these little critters. One will be scooting across the floor like pictured, or hanging on the wall in our garage. At first we just found a case/covering every once in a while, then one day I saw one moving and noticed the worm. I've never seen anything like it and have no idea what it could be or where it comes from. Maybe it's some sort of larva?
The case is tough and gritty and looks made of sand. The one pictured also seems to have some type of fuzz stuck to it. The little worm pulls the case along like a snail with a shell. Since I have no clue what it could be, I don't know whether it is harmful or not. We find them more often in the summer and even then only about once a week. Strange.