Pictures?
It may not be mold at all.
In some instances it is just a buildup of soluble salts.
This is highly common in houseplants grown in inorganic fertilizers (Miricale Gro soil) AND the naturally occurring salts in tap water.
Signs of plant problems that may be caused by excess soluble salts include reduced growth, brown leaf tips, dropping of lower leaves, smaller-than-normal new growth, dead root tips and wilting. When salts concentrate in the soil, water is drawn out of the roots causing dehydration. Excessive salt buildup will eventually cause the roots to die.
Mold would appear fuzzy.
Some pics would really help diagnosis.