Welcome to the hobby!
When you were filling it up, the water surface probably had some sand scum, and it stuck until you filled it up again. Try wiping it.
I have 2 nano tanks, I would remove all bioballs/filtration media. If your tank has 3 compartments, in the first (carbon cartridge filtration) you should put lots of live rock rubble.
In the second (bioball area) you should cut out the black background behind, mount a light, and set up a refugium with chaeto/caulerpa.
In the third compartment with the return pump you can drop a bag of activated carbon to pull yellow organics out of your water.
Just my two cents
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