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Josh different algaes die in different ways. Most will turn a white transparent color and then fade away. Normally thier is not a residue, it sounds more like the algae died, went away and now you have a bald spot with some detritus on it. Blow it off and the algae shouldnt come back to it.
I have both grape caulerpa and Halimeda (sp?) do that. It is a form of reproduction. It happens incredibly fast..so fast that you can almost see it happen if you watch for an hour or so.
Some people are concerned that causes a sudden release of nutrients to the water, but I have not seen anything dramatic come of it.