It's algae growing on the thermometer that the snail on the left can't get in front to clean it. Take out the thermometer and clean it will a towel or a soft tooth brush.
I remember abt a month into the.cycle of my tank in my living room I had turning all my rocks I'm the shade and on the bottoms of my return jets. But after a few days it just disapeared. Its a form of algae. Nothing to worry abt. Check your Mag level and just do your normal maintenance and it'll go away.