Final answer:
The meiosis in the given phrase creates a mood of concern by using understatement to calmly dismiss the severity of a personal infirmity and reassure friends.
Step-by-step explanation:
The mood created by the meiosis in the phrase 'Do not muse at me, my most worthy friends; I have a strange infirmity, which is nothing to those that know me' is one of concern. Meiosis is a figure of speech that deliberately understates the severity or importance of a situation, often for ironic effect. In this case, the speaker is downplaying an ailment or personal distraction, which has likely caused some visible disruption or concern among friends. The use of 'strange infirmity' and the assurance that it is 'nothing' to those familiar with him reveal a calmly dismissive attitude towards his own condition, which aims to reassure or alleviate the worries of those around him, hence the mood of concern is created.