Is ths licjen and if so, how do you treat it?
Yes, this is lichen and there is no need to treat the plant since it's highly unlikely that it's impacting plant health. Lichens are often blamed for the decline / death of numerous plants because they are commonly seen on the exposed limbs and trunks of declining / dead plants. Their appearance is often a result of more sunlight due to thinning branches, exposed limbs, etc. in plants suffering from environmental stress or poor management. These exposed limbs on damaged plants simply give lichens access to the sun they need for growth with little competition. Good plant vigor is the best defense against lichens as heavy infestations are most common on plants in declining or poor health. Following recommended establishment, watering, and fertility practices will promote the development of a thick leaf canopy, which in turn, will inhibit lichen growth on twigs and limbs. Better growing conditions and soil fertility may stimulate new plant growth and ultimately suppress the lichens. Light pruning of affected limbs will remove some lichens and stimulate new shoot growth which may help shade out the remaining lichens. Bear in mind that if the lilac is in extremely poor condition it may not respond to better care and should be replaced.
Thank you for your question.