"They are sensitive and they are completely covered in prehistoric markings which remain to be fully studied and any surface damage to the stones is hugely concerning.
“A rich garden of life has grow on the megaliths, an exceptional lichen garden has grown. So it’s potentially quite concerning.”
This is important - they aren't just a bunch of old rocks, they develop a "varnish" and an ecosystem that can protect them.
They say it's just cornflour and will wash off in the rain but people tend not to coat ancient monuments in cornflour, so we don't really know what the effect might be.