SE Brazil Atlantic rainforest Oct 2017. I'm assuming this is an early stage of an insect. It was crawling around a tree in the moss and was about 6-7mm.
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This is, indeed, a nymph (what you called a young stage), but not from a cicada as you suspected: it's a nymph from a leafhopper, which are hemipterans from the Family Cicadellidae (cicadas are also hemipterans, but they belong to the Family Cicadidae). More specifically, this seems to be a sharpshooter, which are leafhoppers from the Tribe Proconiini.
Narrowing down to the Genus is more complicated, but I'd guess it is Oncometopia. Here is an image of Oncometopia orbona for comparison:
This other image (also Oncometopia orbona) is even more similar to yours: