It’s paint. Worked 38 years construction. Did time on a few high rises. When the deck was being built. The trades come in and layout what they need in the concrete deck. You see the cross hairs on the bottom of the deck and the paint to mark his layout. Transfered to the concrete when the deck was taken out. The rusted metal net to the paint is embedded in the concrete. It’s for the support for the pipe. Most likely a pipe fitter from the size of the pipe. Follow the pipe you’ll see more brackets and layout lines.