That is rubber shelf liner. I only put it over the shaving for their first week after hatching. After that it is difficult and expensive to keep clean. Doggie pads covered with several inches of pine shavings and a non- chicken waterer will be a game changer for you. Sadly, people at feeds...
It is your waterer. You need to stop using a chicken waterer. Most of us use rubber made type containers with holes cut in the top. Setting it on a tray covered in hardware cloth helps too. If they lay in wet, dirty bedding they get ammonia burn.