What is this?

Hello. I have recently setup a 85 litre tank with the help of reading all the great advise on this forum. As I knew I was going to need so much more advise while running this tank I wanted to join the community here.
As a side question. What is this that appeared on my lr on day 3 of cycling? After much researching I still have no idea??? It moves quite slot but I have never actually seen it moving
Thanks

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