Pollen grain after meiosis in the microspore mother cell and has haploid set of chromosomes. A seed develops from ovule after fertilization, in which the number of chromosomes get doubled. Pollen grain is unicellular, microscopic structure, a seed is multicellular macroscopic structure.
Unicellular means "consisting of a single cell". No animal visible to the naked eye is unicellular, though there are single cells visible to the naked eye (like eggs). No lizard is unicellular, no reptile is unicellular, no amphibian is unicellular.