I NEED HELP

I was looking in my sump and I saw a white blob about 4 inches around and 1/2 inch think and I touched it and it was like a big browny gue coverd in a white crust and I just started the tank and only have 25lbs of rock and 1 damsel in it it's a 125 gallon tank and a 55 gallon sump what could it be