This is a shot of a Green fuzzy Mushroom that was about six inches in diameter earlier this afternoon. The only thing different that I did was add plankton to the water. Kind of stringy white substance coming out. Reproduction or becoming food for something?
Somthing iritated it, I would not worry to much, it happens.
If it continues you should start testing water analyzing your practices. Do you put any additives directly in your tank?
