Thank you all. NoodleCats got me. I was asking about the Pearl weed.
I have been able to keep all other plants clean by manually removing occasional green thread algae growing with/on them, however, the pearl weed is so slender that it got entangled with the thread algae, which I am unable to remove without breaking the pearl weed. I hesitate to cut and throw some of the pearl weed since I only have a small bunch.