In [227]: x.mask
Out[227]:
array([[ True, False, False],
       [False,  True, False],
       [False, False,  True]], dtype=bool)

In [228]: np.where(x.mask)
Out[228]: (array([0, 1, 2]), array([0, 1, 2]))

In [229]: yy, xx = np.where(x.mask)

In [230]: yy
Out[230]: array([0, 1, 2])

In [231]: xx
Out[231]: array([0, 1, 2])