I think that may be staghorn algae/
That looks wierd. I wonder what it is? Eggs?
Someone asked about this before. It wasn't eggs it was some kind of algae if I remember correctly.
I have trumpet snails in one of my tanks and I have this same thing inside the canaster of the filter on that tank. I always assumed it was eggs or somthing from the trumpet snails. I don't have it on any of the canesters on other tanks just the one with the trumpet snails.
http://www.monsterfishkeepers.com/forums/showthread.php?527791-Thread-like-growth-on-my-filter
This^ was the other post. If you want to look at it, but I think this was on the outside of the filter.