Seeker is neopagan slang (I've only seen it used in this sense by neopagans, at least) for someone that's looking for a religion for themselves. Seeker usually means someone doing it actively, poring through books and asking questions, though it is occasionally used to refer to someone that is unhappy with their current religion but hasn't been willing to begin looking elsewhere.
Most seekers are looking for a perfect fit, and when (if) this perfect fit is found, Instant Witch Syndrome is all too common. When they finally return to earth, they may be either happy or disappointed in their choice.
Though pagans, particularly Wiccans, are loathe to proselytize, there are scores of articles and references about on, essentially, "How not to scare seekers away"