Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dĭădēmātus, a, um, adj. [diadema], adorned with a diadem: Apollo, Plin. 34, 8, 19, § 79.
Hence,
Dĭădēmātus, i, m., a Roman surname, Cic. Post. Red. ad Quir. 3, 6.