The blue-green slime is not only an ugly sight but also foul-smelling. Furthermore, blue-green algae produces toxins which can be harmful plants in your aquarium. If the slime covers a plant completely, it will not be able to get sunlight and die as a result. So here is a link to ACs website about BGA (Blue green algae)
https://www.aquariumcoop.com/blogs/aquarium/blue-green-algae?srsltid=AfmBOoomnk_8oxhviQiKulfxhYXyD5jrv-na6lzVtcg4e_NfcrwfF7yf
Can be harmful to your fish too, but not super likely.