If you cannot brush it off easily with a toothbrush yes it's most likely coraline. It typically starts off green on rock and then the purple will grow over the top of it. You'll probably notice small purple specks on snails or other plastic spots in your tank.
It’s just algae . Coralline algae takes time and good lighting and a healthy aquarium. If after 6 months no coralline algae then troubleshoot. Get some turbo snails they will help with that green algae.