It's a Sharpie!
Those things are good for everything... I can slap my gangsta tag on the leaves of my plants so all my homies know where my turf is at.
G.
I don't have issues with them, and can say confidently that they do not appear to be toxic to plants, used in that manner, as I've recently HARVESTED a couple from-clone plants that still bore a healthy leaf with the original clone label still visible in black ink!