They are harmless to the other tank inhabitants.
Some considerations though:
1. They can build up inside your underwater equipment too. Powerheads, skimmers, etc. So keep an eye on your equipment and clean them as needed.
2. Avoid touching/scraping them with your hands or arms. The edges could be sharp and different people's skins react to the buggers differently.
3. To a certain degree, they compete with other tank inhabitants for calcium and other trace elements in your water as they build calcified shells.