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If it's been dry for some time I don't think it will be much benefit. But if it's just on the back of the tank and not obstructing your viewing I don't think it will cause much harm either.
If you wanted to remove it soak it with vinegar then scrape with a credit card or razor scraper (it's a glass tank, right?). It should come off fairly easy. Then rinse the vinegar out as well as possible.