The uneaten food is decaying and creating ammonia. Depending on the size of the system and the amount of uneaten food,this can be a big deal -- it's producing more ammonia than the biological bacteria is used to handling, and so it takes a little while for more bacteria to grow, so it could lead to an ammonia spike. The fuzzy stuff isn't "harmful" -- it's just mold or fungus -- but the effect of the uneaten food on your biological filter could be harmful.
The first thing to do would be to figure out a good amount to feed -- obviously, on the day that was fed, you fed too much. Also, uneaten food should be removed (with a net or siphon) immediately so that this doesn't happen. I'd do a good inspection of all of the places uneaten food can float around and get lodged in, and make sure you remove all of it today, and then start feeding less until you see none of this stuff in the tank.