What is this on sourdough starter?
I don't see anything that has grown there, except for the starter itself. The starter used to be more expanded, its skin dried up in the expanded state, then it fell back down and the skin tore in places and wrinkled up - this is what you are probably picturing as a growth. It also trapped some gas bubbles (the round warts). There also seems to be a layer of hooch formed under the skin. Where the skin is torn, a single untrapped bubble is also visible at roughly (45°, 0.2r).
If you had actually had a mold growing, I would have advised against baking with the starter, because it is unsafe, even if the bread rises.