Plant or Pest?

[SunnyD]

Hi All! I recently discovered some sort of being growing in my freshwater tank. It looks more like a pest than a plant but googling hasn't given me any answers, help! What is this thing and should I be doing anything about it? It's now about an inch tall growing on an artificial decoration I have and there's a second one forming on the other end of the tank on a different artificial decoration.

 

[CaptainBarnicles]

never seen anything like it...did you have any real plants growing on it before?