Lilac Bush Problem? I`ve just noticed this in the order of a week ago but...

I`ve just noticed this in the order of a week ago but the lilac bush in our yard have some kind of white substance on it (sorta like moss or cobwebs?). It isn`t on any of the other bushes or plants in close proximity it so I was wondering I should spray some bug spray on it (wouldn`t that hurt the lilac as well?) or is it some helpful of virus or mold?)
Are these plants suitable for an arid climate...
It is an aphid. Treatable next to a suitable wash of diluted washing up juice. Or if you prefer, a spray bought from the local garden centre.

Growing an Apple tree?
I agree with the powdery mildew answer. It's not a big deal, if truth be told. My lilac gets it every year about this time. Better nouns circulation and keeping moisture off the leaves would probably keep the powdery mildew at sound, but it's not enough of a problem to do anything about it.
Rose Eating Bugs?
Souinds similar to somekind of fungus. It may be the mulch? try a table spoon of bleach in a pint of water and spray this on a slect spot or 2 and monitor it for 2 or 3 days. If it disappears its a fungus and go on and spray the whole plant. If it comes final well tackel that when it happens.
Algae prevention?
Answers:    I think what you have is powdery mildew. Usually this doesn't lead to serious damage to the plant other than giving it an unsightly look. The most powerful thing against powdery mildew I know of is Neem oil. You can find it contained by Home Depot and Lowes. Since it is an effective fungicide as well as insecticide, it should give support to even if your problem is not powdery mildew.