This is your male crestie everting one of his two hemipenes (penises) and giving it a clean. Its nothing to worry about and quite common in sexually matures males whose hemipenes sometimes get clogged up as they start to produce sperm plugs, so they pop them out and give them a wash.
You only need to worry if the everted hemipene ever gets 'stuck' so that the gecko seems unable to withdraw it back into his body. You will know when this has happened because the hemipene will be massively swollen, an angry purpleish colour and dry rather than shiny and moist. If you ever find your gecko like this put it into a container with some warm damp tissue paper (this will help to lubricate the stuck hemipene and stop the delicate skin from tearing and bleeding), and take it to a good herp vet ASAP. The vet will either use a lubricant to try and replace the hemipene manually or, if it is hopelessly stuck, will probably amputate it. This isnt the disaster it sounds as the gecko still has another penis left and in theory can still reproduce.
Oh yeah, you may want to double check the sex of your other gecko. If you have a male and female pair then you will need to be on the lookout for eggs from now on. (If you purchased them at the same time from the same place then there is a good chance they are brother and sister, and so should be seperated to avoid inbreeding).
We have a breeding pair and she just laid 2 eggs! I thought it was something like that but I was worried that he got cut or something too. The female is pregnant and she's about to lay eggs soon too, I'm guessing. Shes pretty fat (pregnant belly)! We got them as a breeding pair.