Recurring Slope Lineae (called “RSL”) are seasonally-repeating dark flows that are active at the warmest times of the year. Some of these grow from the top of the steep slope downwards as expected for liquid or granular flows.
Others show different darkening patterns, which suggests different processes. Although HiRISE has acquired many images to monitor RSL sites, it still is not certain how these features form. [More at link]