Depending on how close you want to get to your rocks, you may find it easier to work with just surfaces (no image maps). They'll look crisper and will light correctly if you apply displacement to them.
You might also consider some slope constraints rather than apply them to the entire rock. With the distribution across the entire rock it could be mistaken for part of the rock texture. In most situations, lichen will grow more on a specific face of a rock, reflecting both the amount of light and water that that face receives... but that's not a mask you can generate in TG (yet?)
I'd probably test the lichen on rocks without any extra texturing so you can see exactly what the lichen looks like.
PS I like the images
. The colour of the first, and the distribution of the second.
