No. Bloodthirster (and other sources of Lifesteal) only applies to basic attacks and abilities that work like basic attacks. Morde's passive is considered a spell, and his bonus magic damage on-hit won't be counted into his attack damage for Lifesteal calculations.
If you want Morde's passive (or on-hit damage from passive) to heal him, you'll need Hextech Gunblade instead. Ravenous Hunter and Conqueror are more accessible sources of healing.
If you want Morde's passive (or on-hit damage from passive) to heal him, you'll need Hextech Gunblade instead. Ravenous Hunter and Conqueror are more accessible sources of healing.