Looks like algae. It seems to be right under the light. You might try to reduce the light timing. Or you could add a little screen piece right above that spot on the glass to reduce the amount of light. If you don't have window screen, a piece of plastic needle point canvas is easy to cut and cheap, from a local craft store. It just blocks some of the light.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves