Impact of diploidy on what alleles are seen by natural selection.
In particular, recessive alleles as found in heterozygotes are, essentially by definition, hidden from natural selection and thereby constitute hidden genetic variation. In haploid organisms, by contrast, there is much less opportunity for such hiding, nor even the concept of recessive versus dominant alleles.
Neutral variation, also essentially by definition, too is hidden from natural selection. Phenotypically, however, such genetic variation is not necessarily invisible. That is, just because two organisms are distinguishable such as in terms of their morphology does not necessarily imply that the same organisms differ in terms of their Darwinian fitness.
Note that the expression of basic metabolic processes within gametes, which are haploid, is one means by which otherwise recessive detrimental alleles can be purged from populations. That is, such variation can be "unmasked" particularly within gametes and thereby subject to natural selection.