Welsh has several pluralisation suffixes, of which -(i)au is just the most common. The other common ones are -(i)on and -oedd.
Moron (carrots) is a special case, as the "basic" form is plural - you add a suffix to form a "singulative" - moronen (carrot). Others are mochyn, moch (pig, pigs), mefusyn, mefus (strawberry, strawberries) and plentyn, plant (child, children).
When you talk very generally about a noun, like rice, mashed potato or prepared carrot, you use the basic form of the word, just like in English - the only difference is that the basic form for moron is plural. Which is probably why you're allowed both "carrot" and "carrots".
E.g. "lamb" is oen and "lamb meat" is cig oen. But "pork" is cig moch.
Don't worry too much - Welsh plurals and singulatives are pretty easy to recognise.