Yes, it's safe. The base connector is identical to the original Micro-B, only adding the USB 3.0 data send/receive lines on the side. (USB 3.0 is backwards-compatible with 2.0, and this applies equally to all connectors it introduced – both the full-size and micro-size, type-A and type-B ports).
The end result will generally be the same as if connecting the drive to a computer's USB 2.0 port (which doesn't have these extra pins, either) – you'll be limited to the speed and power that USB 2.0 allows and won't be able to use UAS, but it should still work properly otherwise.
The extra pins are differential signal pairs (RX+, RX-, TX+, TX-, ground) and do not provide power. Compare pinouts for:
- USB 2.0 Type-A/B and 3.0 Type-A/B
- USB 2.0 Micro-A/B and 3.0 Micro-A/B
- The official cable & connector specification is available from USB-IF for free.
Be sure to use a decent cable though, as magnetic disks will probably need the full 500 mA that a USB 2.0 port normally provides. Original phone cables should work fine (modern phones draw several times more anyway), but some cheap replacement cables might be unable to power the HDD at all, or it might repeatedly shut down while in use.