When I was in the Army working on trucks, they had a rating system for leaks in vehicles.
- Class 1 - Film of fluid
- Class 2 - Drip is formed, but you don't see it drop
- Class 3 - You see a physical drip or can definitively state a puddle came from a given spot
The Class 1 & 2 would not deadline a vehicle (deadline meaning it could not be used). A Class 3 is a deadline and must go to the shop to get fixed.
It was this way for all but two different fluids. The first being fuel. The second being brakes. Either of these were automatic deadline items, both because they are huge safety issues.
Most likely, you're not in the US Army, however, I think the same way now (and yes, this is my opinion). If you see any type of leakage coming from the brake caliper, you need to either rebuild it or replace it. The cost of your life, those who may be riding with you, and those around you are at stake. Do as you will, but I'd be fixing it. It's only a matter of time before it fails.