nitrates

finally the battle is almost over after bio pellets and marine pure blocks in my sump and water changes every week I am down to 5 ppm. it started going down good after I started using algone. so I put one in my wife's new fresh water tank,it is now in the middle of its cycle and I went to change the algone and it was covered in a nasty slim and smelled terrible. Any ideas what the slim was.