I'm a newbie as well, and your second paragraph says it all. You're using worm castings, guano and other organic material which almost surely contain nutrients as well. From the pics I have to assume the pot these are in are around a quart. That size of pot with that much material containing nutrients would probably last you a minimum of a month. So at 13 days you fed them nutes and just 4-7 days later(the time it takes most plants to show any symptoms of overnuting) they began to show these signs. I imagine your four other seedlings haven't show these signs because you haven't feed them yet. If you were using say just peat, perlite and another medium that contains no nutes I'd say feed at 10 days, but your soil is plenty hot with nutes so even if you just water to attempt to flush your overnuting you may have issues. Let them dry out completely in my opinion then feed only with water. This is not from experience just basic botany.
