Diatoms. They should eventually go away. The love silicates in the water and grow.
You can use hydrogen peroxide 3% like you would use from the medicine cabinet, and a toothbrush on the decorations. Try 50/50% warm water and H2O2. Scrub off outside of the aquarium and rinse off.
Another option might be additional water changes, less light period and if needed something which removes silicates. I have used PhosGuard by Seachem, there are others.
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