This is an example of phototropism - https://en.wikipedia.org/wiki/Phototropism
Where photosynthetic organisms, such as algae (or products of their metabolism), bore into the rock you end up with phytokarst e.g. http://www.goodearthgraphics.com/virtcave/phytokarst/phytokarst.html ...
You also get constructive phytokarst, especially in cave entrances in the tropics, where stal (of various forms) grow towards the light...