https://www.backyardchickens.com/threads/liquid-mucous-oral.1189537/page-2#post-18796126You mention that you have recently switched to a nipple watering system. Is there any possibility that these chicks are not able to access water now ? Symptoms that you describe can also be caused by dehydration. Water deprived chicks will also over drink and drool when supplied with readily accessible water.
Check out her other thread.
That is exactly what I thought over there too!