So, you take out the key, you notice ectoplasm and you leave it there? Try seeing if any other key has ectoplasm as well. If not, kill the ectoplasm with a
linoleum knife!. I've had lots of keyboards up until today, mainly because I'm very demanding (my keyboard must be white, old, able to survive Michael Tyson and have no additional keys.) It usually works to blow out the dirt that gets into that hole where the protuberant part of the key enters, or to make sure every part in that set is connecting well. Sometimes it kills the keyboard. But science demands sacrifice.