chromo and chromoly definitely mean the same thing. Typically a good steel frame is made of 4130 chromoly where the numbers 4130 actually mean the amount of chromium and molybdenum that are added to the steel to make it better. I am not 100% sure what the numbers specifically are, but I believe that 41 is .41% chromium and .30% molybdenum. Plus carbon and iron to make up the steel.
Chromium is in fact the same stuff that is put on bikes as a top layer that makes them all silvery looking. But, that is a much higher concentration of chromium which makes it silver. A chromoly frame (4130) actually looks kind of gray, not silver, because there is not enough chromium in the tubes to actually produce that silvery effect.
Do NOT confuse chromoly with trimoly though. Trimoly is typically a frame where one or two tubes are made of chromoly and the rest are made with mild steel (high-tensile steel).