This study evaluates the comparative phytotoxic effects of lead (Pb) and cadmium (Cd) on the chlorophyll dynamics and metal accumulation in the moss Taxiphyllum taxirameum. Experimental exposure revealed that Cd induces acute toxicity, resulting in rapid chlorophyll depletion and complete pigment degradation at concentrations ≥200 ppm within 45 days. In contrast, Pb exposure led to a slower but cumulative reduction in chlorophyll, reaching total loss at 350–500 ppm by 60 days. Metal accumulation showed a clear concentration- and time-dependent pattern, with Cd levels rising to 581.6 ppm and Pb to 701.8 ppm over 75 days, indicating a greater retention capacity for Pb in moss tissue. Statistical analyses, including Pearson’s correlation and principal component analysis, demonstrated strong negative relationships between metal uptake and chlorophyll content, with metal concentration emerging as the dominant driver of pigment loss (variance explained: Pb = 72.2%, Cd = 72.8%), while exposure duration played a significant secondary role (~20% variance in PC2). Notably, chlorophyll loss followed a non-linear, threshold-driven pattern, highlighting the interaction between metal burden, exposure time, and physiological stress in the organism. Together, these findings provide new insights into the mechanisms of heavy metal toxicity in mosses and reinforce the value of T. taxirameum as a biomonitor for environmental contamination by Pb and Cd.