The slime is bacteria. Which is essentially what you want. But when you add bio pellets to an existing system that has a heavy bio load, you will get excess slime bacteria for a few days. that is why it is suggested to add the pellets slowly. Also if they appear to be melting, you can slow the tumble in your reactor.
Also if there is an influx of nutrients, like over feeding, or a dead fish, the pellets will react the same way.