I had this happen once, years ago, where through some random glitch, I had a few tickets show up in my account as eCredits for active tickets. I didn't draw the connection and used them on new flights, which then stripped the underlying tickets from the existing reservations they had been linked to. I had to call DL and it took over an hour on the phone for them to unwind it.

The agent had no idea why the ticket value was showing up as a credit in my account, but in short - don't touch it.