мороженое - noun, nentral gender. usually we don't use the plural form:
"дайте мне пять бриктов (шариков) ванильного мороженого" (вместо "пять мороженых")
мороженое (f. sg мороженый, m.sg мороженая, pl мороженые) - adjective. neutral. Affected by frost. Usually applied to food. Мороженые овощи (ones that you should keep in refrigeration at the temperature below zero)
Word "замёрзший" describes either 1(figurative) the feeling of a creature when it's cold any 'cold' (not necessary below 0) or the state of anyhting that ungerwent unwanted changes due to cold -
2 (literal) the _state_ of anything that contains at least some water and became hard. Not necessary the state is unwanted.
Word "мёрзлый" describes something frozen and has negative conntations: you _dislike_ it for this reason. Usually it's ground or food.
"мороженый" differs in that in describes rahter hystory or hystory+state than just current state. And usually is applied to food, that was made frozed intentionally .
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мороженное (мороженный, -аы, ые) - participle!!!!
Not the best part of Russian orthography, sinse the adjectve isn't seriously different from the participle and a lot af Russian make mistakes in the word.
Anyway, if the word contaid prefix (замороженный),
or there're depended words: мороженный by/with whom/what? мороженный where? How? When?
мороженные нами вчера в холодильнике....
,,, the word is considered participle and is written with -нн-. Happily, it's quite a rare case with unprefixed form of this word.