Could anyone please tell me what this is?

I know it has something to do with painting materials onto the terrain. It pops up a few seconds after I start painting a new material. I just don't know what to call it so I can't figure out how to fix it.

I know it has something to do with painting materials onto the terrain. It pops up a few seconds after I start painting a new material. I just don't know what to call it so I can't figure out how to fix it.
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