These appear to be some kind of moth egg mass.  Moths, like armyworms, commonly lay eggs on non-plant surfaces like house siding or even sheets hung out for drying.  There is little one can do to prevent a moth from taking the initiative to lay her eggs on a surface. Fortunately, they are harmless. The egg masses may hatch and young caterpillars emerge after several days, but without an acceptable food source nearby, they will perish relatively quickly.  Simply wipe off egg masses as they appear. Chances are that the egg laying behavior will taper off within a week or two, as moth flights end.