it is lethal when two crested ducks reproduce, part of the clutch will die in the shell due to the lethal gene combination.
Those that survive (crested ducklings) are prone to neurological problems.
here is a thread of a woman battling the birth defect and the neurological symptoms associated with the crested gene.
This is the only duckling out of the four that were ordered that survived. One was dead before it arrived and the other two died due to complications of the crested gene
https://www.backyardchickens.com/t/857143/treating-wry-neck-in-crested-duckling-edited-title
Your duckling you have now will probably be fine, especially since it's not a very big crest. Just don't breed him or hatch her eggs, depending on if it's a boy or a girl.
It only takes one parent to have the crested gene for some of the babies to come out crested and potentially have these life threatening deficiencies.