Correct use of "mooted"

The use of "mooted" here is correct. Your example is a headline - not a real sentence.

Moot in this case means:

verb (used with object)

4. to present or introduce (any point, subject, project, etc.) for discussion.

In sentence form, the headline would include an "is":

Coffee is mooted as a breast cancer preventer.

Meaning:

Coffee is now being presented as a breast cancer preventer.

Since the example is a headline, the missing "is" can be forgiven. The headline is correct.

The English usage of the word moot is:

n. Law . A hypothetical case argued by law students as an exercise.

Having studied law myself, I understand this usage most intimately.

Across the pond, the definition differs:

"The moot was the forum for solving subjects under debate and thus leads to the definition of moot as an arguable or debatable point."

Thus, it seems as though our UK speaker may have meant that the topic of coffee as a preventor of breast cancer had already been discussed.