Following the melting of a glacier, the organisms most likely to colonize the area first are lichen. Lichens are well-suited for colonizing bare rock and are often among the first organisms to establish themselves in such harsh, nutrient-poor environments.
For the second question, a glacier melting is an example of primary succession because no soil is present initially. Primary succession occurs in lifeless areas where soil has not yet formed, such as on bare rock left by a glacier.