It is difficult to be sure what the rash is without seeing it or having a proper assessment.
A possibility could be a patch of eczema or dermatitis resulting from irritation from contact with the mud. This is not scaly but may be itchy. It can sometimes settle with brownish stains or post inflammatory hyperpigmentation which takes weeks or months to clear. Moisturizers and some topical steroids may help to settle this.
Skin infections from bacteria from the mud breaching the skin barrier can also lead to red patches. This is usually red and painful. This is best treated with oral antibiotics.
I would suggest that you see your doctor or dermatologist for a proper assessment to determine the exact nature of your rash if this still persistent.