Answer:
Multiple alleles are genes that have more than two alleles. An example of this would be blood types, with ABO as three separate alleles.
Polygenic traits are traits whose phenotype rely on alleles from different genes. An example of this would be hair type, which relies on genes from different parts of chromosomes.
The main difference is that multiple alleles are genes with 3 or more alleles; polygenic traits do not necessarily have more alleles, but they rely on on multiple genes.