Try this:
0: Close FH if it's open.
1: Open your Height-map.
2: Open your Mask.
3: Copy your Mask, then close it.
4: Set your Height-map to RGB mode, then paste your Mask onto it and anchor it.
5: Use the "Save as" feature to save it as the Mask, you can change it to .png before saving if you want so that it's smoother.
6: Close your image editor, open FH and see if it worked.