Looks like egg yolk. How old are they? Could have been a soft shelled or shell less egg, or they ate the shell before you got there to see this. Since they are all acting OK, I would just keep an eye on them. If these are new layers, then it could just be a glitch as their bodies work things out, or if a bird is coming back into lay after molt, then likewise could be a glitch. I get occasional eggs laid on the poop boards and all I ever find is usually the residue of yolk, they tend to eat up the rest.