  Context: Rocky planets on ultra-short period orbits can have surface magma
oceans and rock-vapour atmospheres in which dust can condense. Observations of
that dust can inform about the composition surface conditions on these objects.
Aims: We constrain the properties and long-term (decade) behaviour of the
transiting dust cloud from the "evaporating" planet K2-22b. Methods: We
observed K2-22b around 40 predicted transits with MuSCAT ground-based
multi-optical channel imagers, and complemented these data with long-term
monitoring by the ground-based ATLAS (2018-2024) and space-based TESS
(2021-2023) surveys. Results: We detected signals during 7 transits, none of
which showed significant wavelength dependence. The expected number of
MuSCAT-detected transits is >=22, indicating a decline in mean transit depth
since the K2 discovery observations in 2014. Conclusions: Lack of significant
wavelength dependence indicates that dust grains are large or the cloud is
optically thick. Long-term trends of depth could be due to a magnetic cycle on
the host star or overturn of the planet's dayside surface magma ocean. The
possibility that K2-22b is disappearing altogether is ruled out by the
stability of the transit ephemeris against non-gravitational forces, which
constrains the mass to be at least comparable to Ceres.
