is it some sort of dissolving tablet that could have put in header tank to, er, mabe clean the coolant galleries, or inhibit rust build up, or a leak fix kinda thing?
looks like broken plastic on the pic, but then thought it looked half dissolved.
if the rad's leaking then i'd be even more inclinded to remove it and check for signs of the solidified white whale sperm.
you say rad's 5 month old? who fit it and where did it come from?