Is it dangerous?

That isn't rust. If it were rust, it would mean exposed metal. If there were exposed metal, there would be sparks every time you use the microwave and it WOULD NOT work fine.

We had one and used it for several years and discovered the teflon paint came off the top and the metal might have been pitted.  Think it was from the steam rising from heated food eventually peeled the paint.  But, it made us worry about where did the paint and metal go?  We never found any flakes and wonder if it was vaporized by the microwaves.  Don't know, but it was the end of the microwave.  There may not be much information about the atmosphere inside a cooking microwave. If you think about it, the usual corrosion reduction chemical equations might not apply for a material being microwaved.

mine is the same.  it's now 35 years old and works fine.  it doesn't become dangerous unless and until a hole develops in the sheet metal.  Don't let rust flakes fall into your food -- grampa

If it's old enough to rust, it's probably time to buy a new one.