A: It sounds like you are experiencing Efflorescence.
Travertine is a very acid-sensitive material and it's highly recommended not to use an acid-based cleaner as it could have a noticeable effect on the surface of the travertine. In some cases, the affect is not noticeable on a highly-textured surface, so it's best to test on a small area first. (DuPont StoneTech Restore Acidic Cleaner can be purchased here: http://amzn.to/1pR3rYD)
If you'd rather try a different route, you can sometimes remove the salts using a plain nylon pad and some elbow grease.