That is the Materia Keeper boss!
Note: if you have an enemy skill materia equiped you can learn 'Trine' from this boss. But to answer your question, you must begin your search for the 'all' materia in this screen by going down the correct tube at the top of the screen. Then, you must jump off the platform that tube lands you on towards the materia in order to nab it. Rather simple, no?
For newbies...that was a FFVII question.