I consulted my dictionary :)...and it says
Anicca/impermanence = មិនអចិន្ត្រៃយ or មិនស្ថិតស្ថេរ
អចិន្ត្រៃយ or អចិន្ត្រ័យ means permanent, indefinite
but would you write Anicca in Khmer as អនិកា? or អនិក្កា? I reckon that the Khmerised word of Anicca would be better suited for a tatoo than the full translation of the term.