This is a form of algae called Nostoc this is found where there is standing or slowly draining water on all types of roofs. This will go into dormant status in the summer months and turn black then so as it absorbs enough water will return to the green slime you explain. I would remove this all in the summer months and treat the area with anti fungicide. Take note that this could be coming from near by trees or from excessive moss slowing the drain of the effected roof. Hope this helped. All the best david.
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