As far as I can see there is no evidence for a link between melanoma and AD.

Formaldehyde is more complicated. There is some evidence that there is age-related accumulation of formaldehyde in the brain and that people with AD tend to have higher brain levels of it that people who do not. There is limited evidence (based on studies of purified Abeta and the various ApoE isoforms) of a causal link.

However, there does not seem to be any evidence for a link between environmental exposure to formaldehyde and risk of AD.