I will have to agree with Dr Irenaeus that it is difficult to diagnose a pigmentary condition without an examination and assessment.
Eczema is an inflammatory condition and often leaves a mark even after resolution. These marks are called Post-Inflammatory Hyperpigmentation aka PIH.
This can be treated by:
- Controlling your eczema (moisturizers, topical steroid creams etc)
- Lightening creams
- Q-Switch lasers
Nonetheless, the first step would be to seek a professional medical opinion.
Hope this helps. All the best!