We can use invisible ink to write each letter of GHOST at each station. Make Invisible Ink To Write and Reveal Secret Messages: Iodine solution reveals invisible ink messages written using corn or potato starch.
Make This Crazy Cool Invisible Ink Marker Using Just 3 Ingredients
Using an erasable pen is a clever idea - I think I read somewhere that freezing the paper would make the ink visible again? (Here: http://www.jetpens.com/blog/pilot-frixion-erasable-pens-a-comprehensive-guide/pt/670 "Because of its heat-reacting properties, the ink may reappear when temperatures reach below -10 C (under 14 Fahrenheit").
How to Make an Invisible Ink Message. Do you want to write your password down so that no one can see it, or do you want to send someone a secret message? Knowing how to make an invisible ink message will have you feeling like a secret.