Answer

If you usually heat things in an open vessel, then there is no difference between a case where the liquid spilled and the usual case, since the microwave itself absorbs the hot steam either way. The difference is only with regard to the plate, and if it is glass then it does not absorb. (But you should check what material it is made of. I assume there is room to be lenient even with modern materials, since they do not absorb in light of the findings of the absorption experiments carried out in recent years.)
In principle, people generally do not use the same microwave for dairy and meat, though some permit it. In any case, it is preferable to use a closed vessel. If one uses a closed vessel, then in principle there is no problem even without koshering. Beyond that, glass vessels do not absorb, but the body of the microwave seems to me to be made of metal, so it is preferable to kosher it by heating an open vessel with water in it (after cleaning). But as noted, this is true even if the liquid did not spill out, and the vessel was simply open.