Combining the Denoising functionality with Caustics can produce unwanted visual artifacts or side effects (e.g. green spots at very bright areas of the image).
Depending on the number of progressive sampling steps a your scene, using Caustics will produce a result similar to the image below. In this example you can see that some of the photons are very bright but seemingly disconnected from the actual area of interest.
Such small but very bright spots are problematic for the Denoising algorithm and can lead to unwanted results as shown in the next image.
To prevent this from happening it is important to additionally apply Clamping to the image. A Clamp value of 10.0 is usually a good starting point to balance the overall brightness and to get rid of the disturbing caustics while keeping the details in the main areas of interest. The following image shows a clamped caustics pattern.
If you combine Denoising with Clamping the results are much better as shown in the final image.