Black spots on acrylic tank walls and decorations?

I've looked, but it seems most sites say it's from plants, which I only have plastic plants.





I put an old media from my last tank in there, a rainbow hidey decoration.





The levels are perfect, but these black spots began to appear on the decoration then started growing on the tank wall.











I keep a betta and a baby African Clawed frog in there.








What is it, and how do I get rid of it?Black spots on acrylic tank walls and decorations?
it can be from ur decorations. as they get old those type things form and just need to be scrubed