With the latest Prism version this shouldn't be the case anymore. Now Prism uses a package file to add the integration to Houdini and the shelf is defined in the Prism installation folder under: \Plugins\Apps\Houdini\Integration\toolbar\Prism.shelf without any hardcoded paths.
Maybe you installed an older Prism version first, which created a shelf with a hardcoded icon path and then you installed the new Prism version to a different location?
You can check in Houdini in which file the shelf is saved to. If it's not the Prism.shelf file then it's from the old Prism version and you can delete that shelf.
With the latest Prism version this shouldn't be the case anymore. Now Prism uses a package file to add the integration to Houdini and the shelf is defined in the Prism installation folder under: \Plugins\Apps\Houdini\Integration\toolbar\Prism.shelf without any hardcoded paths.
Maybe you installed an older Prism version first, which created a shelf with a hardcoded icon path and then you installed the new Prism version to a different location?
You can check in Houdini in which file the shelf is saved to. If it's not the Prism.shelf file then it's from the old Prism version and you can delete that shelf.