2 possible causes: This refers to the headlamp indicator as I'm unsure by description which one you mean?
1there is a small crack in the lense, letting in moisture
2The last time a bulb was changed the seal wasn’t replaced properly at the back of the lamp
The lamp on a C5 in one complete unit and the bumper has to be dropped to take the lamp out, best to get it done in a garage because if you brake any of the lamp clips they cant be replaced and you will need a new lamp (fairly pricey) if done in a garage they will remove the lamp and try drying it out using an air line or heat gun.
Hope that helps.
Brendan,