I hatch quite a few Pekin ducklings weekly. I always end up assisting a few of them. The unassisted ducklings have soft fluffy yellow down but the ones I assist have brown gunk on them that hardens. I’ve tried spraying them with warm water and I’ve tried wiping them with a wet cloth. The cloth works much betbut still not great… can anyone help me? Why do they get it and how can I stop it?