What the yuck?

[Saltwater Money Pit]

New pool here, no plaster yet. They cut the caps off of the return lines yesterday and the PVC is filled with dark clay or rust. Should we be concerned? Where is the water coming from? Any insight would be appreciated, thanks!

 

[HermanTX]

Could they have put some type of leak sealant (like you put in a car radiator)? I would flush all of that out prior to plaster.
Did they do a air with water test on all the plumbing?