Could this word be the indigenous term the Indus Valley Civilisation called themselves, or a variation of it?
I hope you get an answer, I'm really interested myself. In the meantime, try asking /r/linguistics.
Mleccha doesn't only refer to Non-Indo-Aryans who resided in India, but to non-Indo-Aryans outside India as well. It was commonly used to refer to the Afghan, Greek, and Mughal invaders. It's still in use today in many derivations of Sanskrit.
Based on that fact, I would reason that the words are not linked.