Answer: Without seeing the deposit, I'm only offering an educated guess. I could be lichen which is a flat white/green growth and is common and harmless, or your plant could be developing a scale insect population. See if you can pry the deposit off with your fingernail. Lichen can be peeled off and it comes off in flat chunks. Scale insects are oval and individual scales can be picked off. The are rounded on the top and flat on the bottom where they attach themselves to the bark of plants. Lichen can be killed with a lime sulfur spray and it will eventually weather away; scale insects can be smothered with summer weight horticultural oil. The oil can damage the foliage on your lilac so keep it only on the grouping of scale insects.
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