Look at this post it even has pictures, sounds like what you have.
I've been noticing strange black hair-like substances floating in my toilet water. When I flush these "strings" slip down into the bowl from the holes under the brim.
So I checked the tank and sure enough there are colones of black stringy masses flinging to the sides with hairy growth waving around in the water. It looks like algae or something you'd see at the aquarium! It's really nasty. Recently I even saw some red strings.
I have well water that is VERY hard and smells like sulfur. before I had my water softener fixed my water was red with rust, but I never had a problem with this "algae." Now that I have soft water the stuff is flourishing!
My question is: how do I get rid of it in my toilet, and more importantly could this same stuff be in my drinking water even if I don't see it in coming from my sink faucets?
If it is slimy, snotty at the water line and or below the water line, it is IRB or MRB (iron or manganese reducing bacteria). You have to kill it to get rid of it.
If you have an odor in the hot or cold or both waters, you should kill it on a point of entry basis, that's where the water enters the house.
Click Here to learn how to correctly size or program a water softener.
CAUTION, as of Nov 12 2013 all YouTube videos showing how to rebuild a Clack valve have an error in them that can cause damage.