  We show that the simplest universality classes of fracton hydrodynamics in
more than one spatial dimension, including isotropic theories of charge and
dipole conservation, can exhibit hidden "quasiconservation laws", in which
certain higher multipole moments can only decay due to dangerously irrelevant
corrections to hydrodynamics. We present two simple examples of this
phenomenon. Firstly, an isotropic dipole-conserving fluid in the infinite plane
conserves an infinite number of "harmonic multipole charges" within linear
response; we calculate the decay or growth of these charges due to dangerously
irrelevant nonlinearities. Secondly, we consider a model with $xy$ and
$x^2-y^2$ quadrupole conservation, in addition to dipole conservation, which is
described by isotropic fourth-order subdiffusion, yet has dangerously
irrelevant sixth-order corrections necessary to relax the harmonic multipole
charges. We confirm our predictions for the anomalously slow decay of the
harmonic conserved charges in each setting by using numerical simulations, both
of the nonlinear hydrodynamic differential equations, and in quantum automaton
circuits on a square lattice.
