I went to close the AC vent in the apartment we're renting and saw this. Is this mold or just many years of dust buildup? If it is mold, can I safely just remove the vent and wash it?
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Definitely mold. Yes you can clean it off - use a mixture of bleach and water.
I would however be concerned with the original cause. If you don't fix this, it will just happen all over again.