The orange cornflour used to spray the monuments will be washed away by rain, said Just Stop Oil. However, according to the BBC, experts will need to assess the stones to see if lasting damage has been caused. “They are sensitive and they are completely covered in prehistoric markings which remain to be fully studied,” said Mike Pitts, archaeologist and author of How to Build Stonehenge, to the BBC.
That doesn't say they think there is damage, just that they haven't been able to assess it yet.