Its so they can keep track of which baby is which especially when there are multiple clutches involved and when they are being sold it helps the buyer and seller know exactly which one they are getting.
I have never personally done it but many others have so i'm assuming it dries quickly enough that it doesn't hurt them. Yes, it would come off with every shed.
I just set Swordtail's timer for his bath and paused it so I could actually fill his soaking bowl up and he crawled over my phone and canceled the timer
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!