Most importantly, it will have no effect on the performance of your flash as per output, light distribution, etc- it's too small to interfere with anything.

It may come off with an application of Acetone rubbed over the spot with a soft cloth or a cotton ball.

I am not familiar with the speedlight brand in question but I can take a guess at the cause. If that diffusion panel is retractable, when it is not employed it is stored in the housing where it may have come into contact with some of the circuitry. If it is a burn, it may have been caused by coming in contact with one of the internal components that many have been heating up. If it is paint, some kind of coating or melted plastic, that too could have come off of a hot component. There are a number of parts that may have that round shape. Sometimes rapid sequence firing can cause overheating. Even if there is no overheating, there could be some adhesive, insulation, lubricant or other substance that could have transferred onto the plastic.

If the flash is operating normally, not smoking, smelling funny or making crackling sounds, it's probably OK and the black spot remains a mystery.

Jerry's posting about eyeglass retainers raises a question. Do you wear your glasses or take them off, when looking through your camera's viewfinder? I was setting up my wife's Sony HX-400V and she complained the view was blurred, looking through the viewfinder.

"That's because I need to adjust the diopter for you with your glasses on." I said. "But I take my glasses off when I take pictures." she replied.

That made me think a bit...I have my D5500 set up so the view is clear when I'm wearing my glasses which I've always kept on when photographing. Mostly I keep them on out of fear of losing them if I put them down somewhere for the 2 to 3 minutes my memory's good for. Is this a bad technique or simply a personal preference? How about the rest of you? Why? If it's a bad idea to keep them on, I'll go with Jerry's retainer.