Yes, mould (in english - mold) - in any case, fungus - can be a real
problem if shells are not kept in a dry environment.  This can be a
particular problem in tropical areas, less so in temperate climates.
Molds have four basic life requirements in order to grow and prosper -
darkness, warmth, food, and water.  Eliminate any one of these
requirements and mold doesn't grow.
1.  Sunlight kills molds, so shells kept in a sun-drenched area will
not get moldy.  Alas, we all know that sunlight is bad for shells -
it fades them.
2.  Molds grow slowly under refrigeration, and not at all when the
temperature is below freezing.  Not a practical way to protect a
shell collection however.
3.  Molds will not grow on a perfectly clean inorganic surface (like
glass, metal, or calcium carbonate).  They need something to eat.
However, if there is the slightest film of organic matter on the
surface, they will make use of it.  Most shells, after cleaning, do
have some such matter on them.  Also, a shell is not pure calcium
carbonate - there is a certain amount of protein mixed in.  And,
periostracum qualifies as fungus lunch.
4.  No mold can grow in a completely dessicated environment.  Like
all living things, they need water - not a lot of it - just a bit of
dampness will do fine.  The best way to avoid mold growth in a shell
collection is to control the humidity of the area where the shells
are kept.  The drier the better.  If this doesn't come naturally in
your area, a good dehumidifier is your best defence.
Paul M.
Rhode Island