There is no apparent reaction to Lavender's Blue, and nothing reacts to the eye app. It's likely someone very recently removed the chunks from the walls, presumably to remove the graffiti. They may or may not still be here now.

As far as the graffiti, there were multiple writers, either of different sizes or with access to step stools to reach higher up. The writer did not bleed out while doing this. The writing is very old, though it's impossible to tell exactly. Still, it was obviously long before any of the current travelers arrived. The writing seems like a way to mark territory.