I'm far from an orchid expert but I don't think Miracle Grow Potting mix would be the correct medium for any orchid unless it's a terrestrial type that grows in the ground. Moth Orchids are epiphytic by nature and require good air circulation around their roots to do well; heavy soil will suffocate the roots which may be the cause of your plants leaves yellowing.
Here's the American Orchid Society culture sheets for Moth Orchids (Phalaenopsis)"
http://www.aos.org/orchids/cul...
http://www.aos.org/orchids/cul...
We have a few very knowledgeable orchid growers on this site who will be able to offer good advice,
@Ursula @BigBill
