Black looking stuff on my return lines in the overflow box

[RPatsula]

Hello, when I was having someone help me to try and get one of my clown fish out of my overflow box. We notice that there is some black looking stuff all over where my return lines connect to the overflow box. The overflow box is in the center of my 230 gallon tank. We just took the black stuff off and threw it in the garbage. It had the same consistency as mud. I was wondering if anybody knows what it could be. Also none of my lines within the overflow box are glued together. Is it a good idea to glue them together?

Thank you
Rick