What mesapod said is true to a point...
It is probably algae, and yes, some "algae eaters" might eat it. However, your tank is far too small for an algae eating fish, and the fact that it sounds like green spot algae. Most fish don't eat this particular algae.
Green spot is considered "normal" in small amounts, but can be unsightly and a sign that there's an excess of natural materials, waste, food, etc, in your tank. You can scrub the algae off. Regular water changes and keeping your tank out of direct sunlight will help too.
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- Leah -
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