I found nothing doing a search on guinea pig and "clear pus" (in quotes).
In contrast, just "clear pus" has various sites describing either lymphatic fluid or serus fluid. So indeed, if there is some clear fluid present, it may actually not be pus.
Clear liquid from a pimple is 'serous fluid' containing white blood cells killed in the battle against bacteria/inflammation. It is the natural cycle of healing inflammation and white blood cells doing their job.
If it's leaking a clear fluid, it's not pus, and it's not an infection. Sometimes a tattoo will leak plasma–the somewhat clear fluid in your blood.
"When you scrape your knee or something, what is this clear liquid oozing out after it stops bleeding?"
Lymphatic fluid, which is most of the water in your body that isn't blood or inside cells. You have 4x-5x as much lymph in your body as you do blood, but there's no heart to circulate that fluid. Only muscular movement can do that.
So it could be something else that one is seeing.In most cases, clear pus or fluid indicates that a wound is healing. It does not typically signal an infection, and it is usually much thinner than other forms of pus. In fact, clear pus often is not true pus at all. It may commonly be noticed as oozing from healing scrapes or cuts, pimples, and other minor injuries. As long as the fluid is clear, there is usually no cause for concern.