To fully control the Black Sea Russia needs control over Ukraine, Turkey, Bulgaria, Romania and Georgia (in that order). 3 of them are NATO members [0] and the other two are already under western influence. Try to mess with any of them and the III World War will be ignited.
Moldova is almost useless (for that purpose) unless its used to invade Western Ukraine and Romania. They control an impressive 0 meters of Black Sea coast and a whopping 500 meters of Danube's left bank.
The Black Sea is already controlled by Turkey (and NATO). Thank the Montreux convention [1] for that. Turkey's fleet is several times bigger (compared to Russian's presence on the Black Sea) and unlike Russian's they can move freely and expand its capabilities faster. Russia can't even effectively block grain ships parting from Odessa.
Moldova is under Kremlin's radar since its independence thanks to the breakaway territory of Transnistria. The region is a hot mess thanks to USSR policies of changing borders of its republics at whim without caring about the local population. The consequences of USSR's fall did a huge damage to Eastern Europe, Caucasus and Central Asia. Some regions have already recovered like the Baltic states or Poland. Transnistria, like Crimea or Ukraine used to be part of the Russian Empire since they conquered both of them to the Ottoman Empire and the Tatars. Its population was and is a mixture of Ukrainians, Moldovans and Russians. Ukraine and Crimea have their fair share of history compared to Transnistria though. And obviously, there's not an easy solution to this problem.
[0] https://www.nato.int/cps/en/natohq/nato_countries.htm
[1] https://en.wikipedia.org/wiki/Montreux_Convention_Regarding_...