answer:Not much danger unless it is caked on, inches thick. First, the magnetron in the microwave (that creates the waves) is almost always aimed at the center of the oven. The sides get much fewer waves and therefore, stuff on the sides never cooks as fully as stuff in the middle. (Which is why a lot of microwave ovens have those rotating trays, to even out the dispersal of the microwaves) Second (very simply put) , microwaves work on water, fat and other substances that are primarily water or liquid. (of course, there is more to it than I am describing, but for our purposes, this is enough). The stuff on the walls of the microwave has no water content – it is dry splatter. Or more correctly, it had water content when it splattered, but it has no more water in it now. Therefore, the heating waves of the microwave magnetron have nothing to act on. Since bacteria needs to have some level of moisture or humidity to grow, the dried splatter is not a fertile area for it to flourish. So there is no safety issue here. Having said all of that above, it is hard to believe that a landlord would have the gall to rent someone an apartment with a dirty microwave. That’s just tacky. If I went into your place and saw that, I would walk right out and think – “what kind of a schlock landlord is this?” So you may want to clean it to get to renters.