I found this diamond here and the gemologist said it was eye clean, but on the GIA cert there appears to be a mark underneath the table. Would this be really obvious upon closer inspection? Thanks!
Gemologists use different criteria for determining "eye clean." Trust that whatever criteria they used they are being honest. You can ask the dealer their standard is.
It really is nearly impossible to tell without seeing the stone in person. However, if it were me, I would have to think that one primary grade making inclusion that makes the stone as an SI2 would be somewhat visible at 10x magnification. As the previous poster stated, eye cleanliness depends upon the person looking at it and their standard for eye cleanliness.
Unless you are Superman in disguise, you won't see what the loupe sees. Also gemmologists are trained to spot the clues that indicate inclusions, they can be difficult for an untrained person to see, evem with the loupe. If the gemologist says it's eye clean that is a good start. It probably indicates that there is no colour for your eye to pick up.
Just reminding you that eye cleanliness is judged from face-on, looking through the table. It is possible you may catch 'something' from the side, if the light shines through the stone just right. However, there is also the strong chance that you will not, especially if the grade-making inclusion is a cloud. Once set, it may be almost impossible to catch any clear view of the grade-making inclusion whatever.
On paper that looks like a great SI2 to me!