I thought an experiment might be in order.
The white stripes showed up on Stone 56 after Gowland had re-erected it.
Gowland doesn't say if he packed between his "stout timbers" and the stone, but the photo suggests there my have been something placed there.
I have a well lichened Sarsen gatepost, originally from West Kennett, and plenty of stout oak timbers so I have strapped one very firmly to the sarsen and will leave it there for a fortnight to see what happens.
After two weeks I removed the timber and the lichen showed a very small amount of discolouration,
And another two weeks later maybe just a bit more, it looks like lichen has died but it is not conclusive.
I will keep observing.