Its color may be different. Add a little amount of the fluid as it's shown in the image. Wait for a couple of minutes - let the fluid to flow down. Start the engine. Check the level again.
Engine coolant Keep the coolant level full, low coolant level cause the engine overheating.
If it's lower, you should add it, because low coolant level may cause engine overheating and therefore engine damage. If there is no coolant in the expansion tank at all, you should add the coolant into the radiator also.