AC voltage on the outside surface of micro USB charger cable
This voltage is due to standard leakage between primary (AC side) part of AC-DC converter, and secondary part. The effective impedance of this parasitic leakage is about 100k - 1000k. The leakage is allowed to be from 75-100 uA for good medical grade PSU. Conformance to UL 60601-1 dictates that the maximum allowable leakage current is 0.3mA. This kind of current is not harmful, and easily gets grounded when the plug is inserted into device. However, high-impedance probes (oscilloscope or good DMM) will show this phantom voltage when the charger is not connected to anything.

As I understand, the coupling (which causes the leakage) is intentional, because fully isolated supplies might built up substantial DC voltage if left floating, which can be really harmful.