They will not be affected by the monster attracting spell since they didn't dip into the hot spring fully.

However, since Levi put his hand in completely there's a strong possibility of his fingers growing back in two month's time. The spring has secret magical properties (also discoverable by testing the sample water) that grow back lost body parts. These parts will be a different color (of your choosing), but they will function as if they're the original parts. Think of them as magical, highly advanced prosthetics.

The fingers, however, are magically made, therefore curses and things that nullify magic can cancel the healing process or completely reverse already healed body parts.