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It's just algae because of the light hitting it. It's actually a good thing. It means your sand is full of nutrients and beneficial bacteria. Usually takes a few months for sand to start doing it so good job.
Jmk is right on that. Light is reflecting off the glass in to the sand bed in the front. If you hate the look you can stir the sand. Just know you will kill life in the sand when you do. But where there is death there is light