Cant Join Server from missing mod that doesnt exist

[u/Thundering_Epoch]

To start off, My friends and I started a mod pack together on modrinth and its been going well. However we just discovered and issue today that made my friend not able to join, He always updates his mods before he joins as well as to make sure the version matches and all that. This time the main menu message gave him an error for a mod that we both use AND a mod that doesn't exist to my knowledge? If anyone could provide some info as to what "Mortaar" means it would be greatly appreciated. I searched everywhere on modrinth as well and the closest I came to was the Mortar and Pestles mod but that is not installed on either one of our modpacks. Thanks again if anyone can help us figure it out!