mustard / mustered are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
mustard: (noun) any of several cruciferous plants of the genus Brassica. (noun) pungent powder or paste prepared from ground mustard seeds. (noun) leaves eaten as cooked greens.
mustered: (verb) gather or bring together. (verb) call to duty, military service, jury duty, etc..
mustard: m·AH·s·t·er·d
mustered: m·AH·s·t·er·d
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)