if it is actually on the white part of your eye its probably a phlyctenule.
http://en.wikipedia.org/wiki/Phlyctenule
http://www.opt.pacificu.edu/ce/catalog/13890-AS/Fig%2022.jpg
the atricle above says phlyctenule is caused by
staphStaph aureus food poisoning, but i ahve also seen them occur with excessive RUBBING.
anyway, they are not dangerous. usually self resolving with no medication. if you have been rubbing it, stop. if it persistes past a few days or worsens, go to the eye doctor. even if it worsens and needs medical treatment its still not dangerous...only annoying.