Red offline clips

i'm working inside color and for some reason some of the clips have the 'offline' red mark. I know they aren't and they actually do show up in the viewer, but i wonder if this is some other type of warning?
thanks

while i'm here i'll ask another.. is it possible to attach a 'custom shape' mask to a motion path? The 'use tracker' feature won't highlight when you create your own mask even though i've already tracked the path.
double thanks
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