The Russians are already occupying part of Moldova.
Moldova used to be one of the Soviet republics. When the Soviet Union broke up, it had a short civil war. There is a strip of land in the eastern part of Moldova, bordering Ukraine, on the east side of the Dniester river -- this was the most Soviet/Russian part of the republic, it hosted a large Red Army presence and lots of industry. People in this part of Moldova (Transnistria) preferred to stay part of Russia. They were worried that Moldova might try to reunify with Romania (there are a lot of historical and cultural links).
So in 1991 there was a short civil war which ended in stalemate. Ever since then Transnistria has been a de facto state, which cannot be reclaimed because there are several thousand Russian troops stationed there to guard its autonomy.
It has been run mostly as a mafia state, completely corrupt, depending on huge organised crime funding to survive. BUT in more recent years it has been trading more with the EU, and people think in the long run the conflict can be resolved through closer economic integration.
Russia has never wanted to annex Transnistria but it is a hugely convenient base for them and they will not want to give it up either.
If they can take Odessa and all of southern Ukraine then they can attach Transnistria to this territory. I do not think they would try to take the rest of Moldova. They would not gain that much and it would cost a lot, and there is not the same historical/cultural pull.