They are probably planaria. They are harmless and won't harm your fish or anything.
Both planaria and algae can be a sign of overfeeding and/or too much uneaten food and debris build-up in the tank and gravel. I would do a good water change and a gravel cleaning (if you have gravel) and be diligent about removing any uneated food or waste.
Both planaria and algae can be a sign of overfeeding and/or too much uneaten food and debris build-up in the tank and gravel. I would do a good water change and a gravel cleaning (if you have gravel) and be diligent about removing any uneated food or waste.