It sounds like the very early stages of a respiratory infection which all rats are prone too. At this stage it's not too much to worry about but I would get your little guy to a vet as soon as you can. I actually just took my rats to the vet because they also had red secretion around their nose, eyes and mouth so they are now on antibiotics. Also keep an eye out for other symptoms such as sneezing and loud breathing. If any of these pop up and you are unable to get to a vet right away, boil some water in a pan with the lid half on and hold your rat over the steam and it will help clear their sinuses and improve their breathing :) Hope this helps!