Well moss and lichen are generally just indicative of good air quality and appropriate moisture and light levels for that species (plus probably some other co-factors).
As far as I've been able to tell its not really indicative of anything other than good growing conditions for that moss/lichen.
Although many lichens grow on trees, there doesn't appear to be much that it says about the trees themselves. Some lichens grow preferentially on some tree types but that seems to be mostly due to the type of habitat the tree provides for it. The trees are used as a support structure and there may be some minimal usage of minerals, etc.. from the bark but they don't actually attack the underlying structure of the tree (unlike mushrooms or fungal infections which are indications of either poor tree health or an aggressive fungus depending). Some lichens will slowly eat rock and other similar substrates but the rate is really really slow so the trees tend to re-generate the bark layer much faster than that.
There is some evidence that there may be some cross benefit though as lichens and moss will grab onto airborn moisture that could otherwise be lost.
There may be some other cross species interactions I'm unaware of though as lichens are actually incredibly complicated little things. For instance it was just recently (within the last couple of years) discovered that lichens are actually three species in most cases there is also a yeast involved as well as the traditionally known fungus plus alga of cyanobacteria symbiosis (the yeast turns out to be a critical element in some of the lichen based dyes - which were made for thousands of years)