Using multi-wavelength observations from the Solar Dynamics Observatory, we investigated the six-month decay process of the solar active region (AR) NOAA AR 12738 from 2019 April to October. We systematically analyzed the region’s evolution by examining extreme ultraviolet intensity variations, quantifying magnetic flux diffusion, and investigating thermodynamic changes via Differential Emission Measure (DEM) analysis. This study presents the first long-term tracking of a peripheral dimming region (dark moat), revealing its continuous areal decrease over time. DEM results reveal cooling plasma signatures and thermal restructuring, with the dimming region exhibiting a distinct temperature deficit in the range 105.5–105.9 K. Potential field extrapolation identifies two dominant magnetic configurations: low-lying loops with cool plasma (<105.5 K), and high-arching structures connecting to the AR core, contributing to localized emission reduction. We found that the dimming is dominated by high-lying loops extending from the AR core, which are heated to temperatures above the main response of the 171 Å passband (>105.8 K), consequently lacking plasma at the typical 105.8 K formation temperature. The thermal deficit, not just the absence of material, is the key driver of the reduced emission. Our results demonstrate that long-duration dimming provides a valuable diagnostic for understanding AR decay, thermal evolution, and coronal magnetic restructuring.