In "Roman Fever," why are Mrs. Slade's black brows drawn together "as though references to the moon were out-of-place and even unwelcome"?

There is a sense in which the moon is symbolic in this excellent
short story. For these two middle-aged women who are now widows, and are only able to live the
joys of youth vicariously through their daughters, it appears that the moon is a symbol of what
they have lost or passed through their age, and perhaps it is also a symbol of the way in which
Alida Slade tricked her friend into meeting her husband at night in the
Forum.

Note the way in which Alida Slade says, somewhat
disparagingly:

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"Moonlight--moonlight! What a part it still plays. Do you
suppose they're as sentimental as we
were?"

Alida here refers to their
daughters and the way that moonlight is associated with romance and love. Thus the moon seems to
operate as a symbol that recalls their own youth and the role that romance and passion played in
their lives, and simultaneously the way that those days have passed for them now and can never be
reclaimed. This is why Alida Slade greets references to the moon with such a
response.