Air carrier Captain reported an unknown white power on floor of 2R lavatory during taxi. Aircraft returned to departure gate per QRH procedures where white powder was cleaned up by cabin cleaner and signed off by MX. Captain stated a lack of a clear procedure regarding unknown substances or possibly Hazmat items be instituted.

Narrative

After pushback from Gate XX; Flight Attendant (FA) came to cockpit with cell phone photo of white powder on floor of lavatory. FA asked last passenger that used lavatory about the white powder. Passenger did not cause the powder mess. Our QRH requires a return to the gate. After multiple unanswered calls to Operations; we asked Ground Control to help us taxi and return to Gate XX. Ground Control sent truck with marshaller to park us. We were met by contracted [company] Agents and contracted Maintenance. They sent for one cabin cleaner to clean the lavatory floor. This cleaner used her bare hand to swipe and smell the white powder. She then cleaned the floor. Maintenance signed off the AML Unknown Substance entry as 'cleaned lavatory'. We departed for ZZZ.No police; fire; or rescue personnel came to aircraft.My concerns are: 1. What should be the proper station response domestically and internationally to reports of unknown possibly hazardous substances aboard an aircraft? 2. I almost had to evacuate the aircraft on the ramp due to lack of response from [company] contracted employees. 3. Should the passenger have been interviewed/isolated to determine who created the possible hazard? 4. Not knowing individual country procedures; should I have evacuated the aircraft and requested emergency response assuming worst case scenario? 5. Not knowing the exact substance present; is there a defined proper procedure to clean the surfaces to avoid follow on contact illness.[The Cause was] lack of corrective procedures concerning event[Reporter Suggested the need for a] define proper procedures and improve communication between ground personnel and crew.