After decades of searching, the true nature of dark matter still eludes us. One potential probe of the form of dark matter in galaxy clusters is to search for microlensing variability in the giant arcs and arclets. In this paper, a simple method is introduced to characterize pixel variability in the limit of high optical depth to microlensing. Expanding on earlier work, the expected microlensing signal for two massive clusters, A2218 and A370 is calculated. It is found that the microlensing signal depends sensitively upon the mix of smooth and compact dark matter in the cluster. Comparison of two deep exposures taken with James Webb Space Telescope or 2-h exposures taken with a 30-m class telescope in two epochs separated by a few years will possibly detect a few dozen pixels that show strong variability due to microlensing at the 5σ level, revealing a wealth of information on the microlensing population.