The moniker "Bulgar Slayer" is likely a misunderstanding. There's no evidence of Attila actively targeting or engaging in a significant conflict with the Bulgars during his reign.
However, it's important to remember that the Bulgars were a nomadic people who often shifted locations, and their interactions with Attila and the Huns were complex and varied. Some historians speculate that there might have been clashes, but there's no conclusive evidence to support the idea of a major conflict that would earn Attila the title of "Bulgar Slayer."