This letter first identifies a new mode of lowfrequency harmonic instability when combining grid-forming control with the typical DC voltage droop control necessitated in multi-terminal high voltage direct-current transmissions. This mode does not exist when grid-following control is applied and its mechanism is explicitly revealed by the DC-side input admittance modeling of the converter. Then, a phase compensator plus a virtual power system stabilizer are introduced to pacify the input admittance within the critical frequency range and de-risk the related instability. Last, it is also demonstrated that the proposed control strategy achieves excellent performance in contingent active power imbalance ride-through in the DC system. All the analyses are verified by electro-magnetic-transient simulations with a modular multi-level converter model comprising 278 submodules per arm.