Answered Layer View in Mattercontrol has odd symbol?

Hi again friends,

I am preparing a print but when viewing in layer view, I see this odd symbol that is like a little red circle or stop sign?? There are multiple, i've included pictures of the big one and one small one, there are a few small ones, but its the big one that caught my eye...It is not part of the print, I am interpreting it as a sign to tell me there is something wrong on that layer in that area...but I have never seen it before, does anyone know what it is?

Thanks!!

 

Also, it seems like this file is skipping some layers in the perimeter...like every so often there will be 3 layers of support material where the perimeter should be solid. Here's an example where the green lines are support material but you can see the solid perimeter below them and it starts again in a few layers. Is this the result of settings or more likely an issue in my mesh? Thanks!!!