If your newt was recently-acquired from a pet shop, then this is a common problem (usually referred to as limb rot). It is an infection spread during transport/sale of wild-caught amphibians.
If your newt has been with you for a while, then the problem is more likely due to serious problems with water quality. In this case, get the newt out of the water immediately. Place on damp paper towels in a secure plastic tub until you get the water quality problems worked out.