I always called that perliteSeedlings turn yellow if they grow in acidic or saline soil, as indicated by whitish spots protruding on the surface of the soil, which are formed from hard water. It is best to water the seedlings with rain water.
And put the clear one in an opaque cup. You don't wanna risk algae growth.Overwatered, weigh the cup, if its over 175g shes drowning. Next time use a clear cup to see how roots and water travel.