Can you post a photo? It is very likely lichen, which is completely harmless to the plant it is attached to - it just uses it as a support system to obtain sunlight and atmospheric moisture. Lichens can appear on completely healthy trees as well as those which are stressed. They tend to be a bit more obvious on stressed trees, as a thin or sparse canopy allows for better light penetration and faster lichen growth. Lichens can grow on many different surfaces other than plants including rocks, fences and other structures. I've even seen lichen growing on abandoned or junked cars!!.

In general, lichens are less common on conifers as the dense foliage of most makes the tree less hospitable to the development of the organism. But it is important to remember that the lichen themselves are harmless to the host plant and are NOT contagious. The presence of lichen is typically an indication lower levels of air pollution and a reasonably good quality of atmospheric conditions in the neighborhood.

as to what gal said.. see link

https://www.google.com/search?q=Lichens&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi0nL6ro-zPAhXCOCYKHSotBi8Q_AUICCgB&biw=922&bih=745&dpr=0.9

malus .... which includes apples... is subject to all kinds of problems ... as indicated by how orchards have spray regimens ... but generally speaking.. i am not aware that any of them would cross species to bother conifers ...

that is a broad generalization .. and i would welcome any correction if i am wrong ...

but my point is.. as fas as i am concerned... you are way over thinking it.. go for it ...

ken

Thanks, gardengal and ken, it is lichen according to the
Google images, sorry for the ignorant question. Yes, I am perhaps also a bit anxious
because I will be cultivating a corner of the neglected backyard next spring as a
starting point for a multi-year effort. One option for starting a knot garden LOL is described here and I want to make sure that everything goes well :-)

"but generally speaking.. i am not aware that any of them would cross species to bother conifers ..."

One does see lichens on conifers here. The lichens themselves are not at all fussy about their supportive hosts and will grow on and colonize anything that offers the conditions they prefer. In general, because of the usual density of their growth habit and the permanence of the foliage, conifers tend not to make an ideal host plant for the lichen. However, in stands of native conifers here in the PNW, you often do see them growing on lower, mostly bare branches or in stressed trees that do not have a dense canopy. Struggling western hemlocks are a frequent host :-))