Is it valid for the Imam to supplicate in Ramadan by saying: "O Allah, O Purifier"?

There is no harm in supplicating to Allah Almighty using descriptions derived from His actions, such as your saying: "O Allah, O Purifier of the hearts of Your allies, purify our hearts," and "O Turner of hearts," and "O Disposer of hearts," even if these are not among His Beautiful Names. This is the view of the majority of scholars, and it is what the predecessors of the Ummah and its imams followed. The Permanent Committee has explicitly stated the permissibility of saying "O Helper, O Facilitator, O Granter of Ease, O Lord," because the intended meaning is Allah, Glorified and Exalted be He.