Not necessarily. The rock may only be a few centimetres underneath the sand dune, in which case it would not appear to protrude out from the dune significantly. There is also the possibility that the "spiders" are black sand[0][1] or silt, as they look suspiciously like some formations on Earth (albeit on a much larger scale):

[2]: This is a sand dune on a sand island, but has some black discolouration from rock just below the surface. I'm not sure how far down this rock extends, but its visibility varies depending on the time of year and number of visitors to the island. Note that there is also some grass (brown) and other vegetation (green) around the edge of the clearing (particularly on the southern edge), which is difficult to distinguish from the black rock.

[3]: Same island, different clearing. There is still some vegetation in the picture, but it's easier to see the black sand and rock underneath the top layer of white sand.

Try playing with the time slider at the top of the page to see how the exposure of the rock changes.

[0] https://en.wikipedia.org/wiki/Black_sand [1] https://en.wikipedia.org/wiki/Heavy_mineral_sands_ore_deposi... [2] http://www.nearmap.com/?ll=-27.194111,153.379091&z=21... [3] http://www.nearmap.com/?ll=-27.280641,153.410605&z=20...

I see some shadows in the bigger dune at the bottom-left (crossed by the yellow line).