I also see leaf yellowing on hoyas that don't like cold in the winter. I think Eriostemmas fall into that category. I have one that last year did this while being in front a window, found later that the window leaks air and it was getting a chill. This year I moved into a warmer area and very little yellowing or leaf drop. Could also be a deficiency as was pointed out, I don't fertilize at all in winter and water about once a week. I keep them on the dry side since they are kind of hibernating in the house.
