That appears to be mold and you should get rid of it. I've seen similar methods online, but fruit, water, and sugar appears to be a method of making a starter for home-brewed alcohol rather than sourdough bread.
To make a sourdough starter using fruit, you should use flour and water and remove the fruit. The yeast are only present on the surface of the fruit,...
The steel bowl gives you a precise ability to heat or cool the dough as necessary. If you have an infra red thermometer you can just find places in the house that are warmer or cooler. Us the fridge or a warming mat if necessary.
Most bread recipes that have cornmeal usually use a combination of cornmeal and wheat flour. I have made pizza crusts with cornmeal leavened with yeast, but they are about 1/3 to 1/2 cornmeal. Any more than that and they have a texture more like polenta or grits than bread. Short answer, yes, if you mix the cornmeal with wheat flour.
Based on your response to my comment, it appears that you have undissolved salt in your dough. I've seen this before. You can use kosher salt when baking bread, just be sure to add it at a stage when there is enough liquid to dissolve it.
Try feeling the texture of the spots. If they are clumpy when you press down on them, it might be a case of flour clumping together when mixing. Otherwise, if the texture is smooth, it might be something else.
It's impossible to say what it is - but I am quite sure what it isn't. I have never seen or heard of a pathogen (mold or otherwise) which is able to build visible colonies during such a short time at room temperature, especially in the presence of yeast. And your yeast was not dead - the dough rising proves it.
This is almost sure some ingredient not being ...