These typically form due to sublimation, where a solid transforms directly into a gas, in this case, carbon dioxide ice. When this occurs, it disturbs the surface and exposes the darker material underneath. Other formations can take on the shape of small, radial channels that look more spider-like to our eye.
This observation features an amazing variety of surface terrain and araneiform structures that we dub “spiders.”
Image: NASA/JPL/University of Arizona https://www.uahirise.org/hipod/ESP_065230_0995