Do the isolated pockets of Greeks in Russia have a dialect very different from Standard Greek?

A2Q (as opposed to A2A) by Peter J. Wright.

There are two Greek dialects spoken in the former Soviet Union.

The larger population speaks Pontic Greek, spoken in southern Russia, southern Ukraine, Georgia and Armenia. The population is descended from Pontic Greek speakers from their original homeland, on the southern shore of the Black Sea, who moved to Christian Russia from the Ottoman Empire in the 18th and 19th centuries. Despite what the linked Wikipedia article hints at, I don’t know of any linguist that speaks of their dialect as anything but Pontic. Those in Tsalka are Turkophone.

Pontic Greek is not really mutually intelligible with Standard Modern Greek, although you can pick it up as a Standard Greek speaker relatively easily.

The smaller population speaks Mariupol Greek, in the villages around Mariupol in Eastern Ukraine. The population originally lived in the southern Crimea, and moved to Mariupol in 1778—again, moving from a Muslim realm to Christian Russia. A substantial number of Christians, who moved to Mariupol itself, spoke a variant of Crimean Tatar known as Urum, and the Greek dialect is substantially influenced by Tatar. The dialect itself is distinct from Pontic (although some of the villages surrounding Mariupol, like Makedonovka and Anadol’, are in fact Pontic-speaking); it is somewhat less divergent from Standard Greek, but if it’s mutually intelligible with Standard Greek, it’s only barely.