hi
yoyos & other loaches will turn white or have their markings fade when excited or it can be if they are stressed or whilst having a fight and you'll often see this behaviour when a new loach is added to a tank. ive also seen it once after adding a medication to our tank.......supposedly ok for loaches & i did add the correct dose, both our yoyos went white for a few minutes which was enough for me to do a quick water change as it was obviously bothering them.
its nothing to worry about as long as all your tank parameters are normal (ammonia,nitrite etc etc)