I think that the Eucharist is valid in a Mass at a Greek Orthodox or Russian Orthodox Mass. Am I correct?
Yes, the Communion of the Orthodox churches is valid because they have maintained apostolic succession and have a valid priesthood. As an aside, Orthodox and Eastern Catholic churches do not use the term Mass for their eucharistic liturgy because it is a Latin word and thus associated with the Roman Catholic church. The Orthodox and the Eastern Catholics prefer to use divine liturgy.