Mutations on the COL6A6 gene, which is responsible for the synthesis of collagen, correlate very strongly with eczema. Collagen helps to hydrate and provide relief for eczema

Collagen is good for hydrating the skin.  But Eczema has no cure.  The best you can hope for is to manage the flareups. 

From National Eczema,

Collagen is a naturally occurring protein in the human body that makes up around 75% of our skin. According to Dr. Shari Marchbein, a board-certified dermatologist in New York City, “collagen is the main structural protein that forms the connective tissue throughout our body, from skin to bones, muscles, tendons, and ligaments.” Within our skin, collagen also helps maintain hydration – a critical concern for people with eczema, as prolonged dehydration of the skin can lead to cracks and fissures. Skin dehydration is one of the causes of skin barrier dysfunction in people with eczema and may increase the risk of Staph infection

from Dr. Harlan, 

Collagen synthesis is clearly an important part of the skin’s health. But can this “glue-producing” protein really help eczema?

In a word, yes. Collagen is an important part of your skin’s barrier function. Inadequate collagen synthesis can lead to problems ranging from dryness to poor healing. Mutations on the COL6A6 gene, which is responsible for the synthesis of collagen, correlate very strongly with eczema.  

Adding that, 

A 2017 study led by the Japanese Institute for Investigative Dermatology found that eczema patients taking collagen tripeptides had lower blood levels of inflammatory cytokines and chemokines than eczema patients who did not supplement. While the study didn’t definitively prove that collagen supplements help eczema flares, it suggests that they could, if given within the right parameters. 

Sounds good on paper,

In 2022, a Canadian study found that marine collagen, which is taken from fish, jellyfish, and sponges, contributed to improved skin barrier function and faster healing of the skin. The reason for this seems to be that this kind of collagen stimulates the migration of fibroblasts. We already know that fibroblasts play a key role in collagen synthesis; they also act as “flying squads,” migrating from their source to deal with damaged tissues. This migration is inhibited in eczema-prone skin. Marine collagen seems to enhance the dermal fibroblasts’ ability to migrate from the dermis to other layers of the skin as needed during eczema flares. 

This could be the difference, 

The study’s authors note that marine collagen peptides have a smaller molecular weight than most traditional bovine collagen supplements. This means that they are more easily absorbed by the body’s tissues.