Lower Unit Gear oil looks like eggnog!

[JohnR]

Very well could be... Any oils that have a milky look to them indicate water getting in - much like a headgasket.

Get it checked and service right away. One thing you can do if you service it yourself is to use a extremely clean drain pan, drain the gear oil, and put a strong magnet in it and run around for filings - excessive filings could indicate a severe problem (some drain plugs actually have magnets built in to trap filings from causing greater wear). Either way, best to have a good outboard mechanic check it out. If the fluid levels are still decent, you might not have any problems after changing the fluid and the seal.