In Foldit, "mutable" means "capable of being mutated". In design puzzles, one or more segments are mutable, meaning they can be changed to another amino acid. See mutate.
Some spell checkers don't recognize "mutable" as a word, but the Oxford English Dictionary has this citation from 1646:
There is no East and West in Nature; nor are those absolute and invariable, but respective and mutable points.
Thomas Browne · Pseudodoxia epidemica · 1st edition, 1646 (1 vol.). London: printed by T. H. for Edward Dod
Somehow that seems appropriate. Foldit has a few mutable points.