Everythingg
King-Over-Kingz
There's a very good chance they'd have mathematics we wouldn't understand, but remember that it's all based on basic concepts. There's no math so advanced that it erases our understanding of geometry, for example. Because all mathematics is, is an extrapolation of the natural laws of the universe. For example, if you have two of something on earth, and you get two more of it, then you now have four of something. That is also true on whatever planet they come from. True, aliens wouldn't understand if I drew 2 + 2 because they have no reference for what the symbol '2' represents. That's why we'd probably use simple geometry to start. So if I drew a circle and then a line halfway through it, I've demonstrated that I understand the relationship between a circle's circumference and it's radius (pi). If they have circles where they come from (the single most basic shape in nature) and if they're advanced enough to get a spacecraft here, then they've undoubtedly discovered the same ratio, and voila, we now have a common understanding and we've identified ourselves as intelligent beings.
Let me ask though. You're not saying we are going to communicate wit these (demonic) beings using math right? If not then how is that the universal language?