Air conditioners, particularly split ductless air conditioners, tend to have build-up of bacteria and mold colonies due to the condensate which is the water that condenses onto the cooling fins much like the water which forms around your cold beer can.
Over time, this moisture breeds bacteria and mold which would grow into colonies. The Slimy substance is what we call bio-slime or bio-film which is the byproduct waste of these bacterial colonies. They eventually clog up your cooling fins and drainpipes causing significant drop in airflow and leakage in condensate.
How To Clean Out This Slimy Substance in Less Than 20 Minutes From Your AC?