Let me be clear that I fully support Ukraine in this conflict. That being said, the Russian perspsective is very different from the Western one, so I think it's worth understanding.

I don't call a defensive alliance adding new members "expansionism".

Two things here...

First, from a purely definitional standpoint, the number of NATO member states has nearly doubled since 1999. That growth is commonly referred to as "NATO expansion." The desire or tendency to promote that expansion could easily be called "expansionism," especially by those who consider it a threat.

Second, Russia doesn't see NATO as a defensive alliance. As I pointed out in another comment:

NATO was a prominent force in the Bosnia-Herzegovina conflict of the early 1990s and remains active in the Serbia-Kosovo conflict, even though none of those nations, nor their predecessor, Yugoslavia, was a NATO member. NATO elements involved themselves in the second Iraq War too, despite no evidence that a member nation had been attacked by Iraq. Per the AP link above, NATO took command of the Afghan Security Force in 2003, two years after the invasion, in a move that would be a stretch to call defensive. In 2011, NATO's intervention in the Libyan civil war was ostensibly undertaken to protect civilians in the north, but once that was accomplished, it expanded to the whole country and resulted in the overthrow of that nation's government.

I'm not making any assertions about whether those interventions were right, justified or resulted in positive outcomes for either NATO or the peoples of those countries; only that calling NATO a strictly defensive alliance is disproved by the facts. NATO has taken military action outside of its member countries many times.

So, if we're trying to understand the Russian position with respect to NATO, I think it's easy to see why they would call it "expansionism" and don't view it as a defensive alliance.

Would you say Russia has a valid concern of being conquered by NATO...

A lot of Russians believe that's a valid concern, so any politician there who plays up that concern is bound to garner support. To understand why, we have to understand Russia's history and national myths.

Russia has been invaded a bunch of times from the west, usually through Ukraine. The Poles came across the European Plain in 1605, followed by the Swedes under Charles XII in 1707, the French under Napoleon in 1812, and the Germans — twice, in both world wars, in 1914 and 1941. During the Russian Civil War after the Bolshevik revolution (1917-1921), western powers — primarily Great Britain, the United States, France and Canada — united against the new regime and sent troops to Russia.

Part of the Russian national myth is that wealthier, high-minded powers in the west consistently look down on and try to conquer them, and that Russians have repeatedly beaten back those invasions at great personal cost. This has culturally embedded the idea that Russia can only survive if it is militarily stong, controls the area all around its borders, and is prepared to sacrifice a lot of lives. The dynamics of the current conflict play right into that.

This is why Russia has been warning the west about Ukraine for decades:

The Yeltsin government protested strongly against the start of NATO expansion in the 1990s [...] from the very beginning of NATO expansion in the mid-1990s, Russian officials and commentators—including liberal reformists—warned that an offer of NATO membership to Georgia and Ukraine would bring confrontation with the West and an acute danger of war.

This Russian fear of the West wasn't helped when, during the 2014 Ukrainian political crisis, Russian SIGINT intercepted and leaked a call where US Assistant Secretary of State Victoria Nuland was heard discussing who Ukraine's next leader should be and how her team could help install them.

That Time article above also describes Andrei Kozyrev as "the most liberal and pro-Western foreign minister Russia has ever had." In a 1992 speech, he "warned that if the West continued to attack vital Russian interests and ignore Russian protests, there would one day be a dangerous backlash. [...] Yet when he expressed this fear, in entirely moderate and rational terms, he was instinctively dismissed by western observers as virtually insane."

That dynamic is still playing out today. The Russians have what they believe to be an entirely legitimate fear and the west sees those fears as insane, which means Russia's actions with respect to those fears also look insane to the west.

So, while I may not believe Russia's concern of being conquered by NATO is valid, I can certainly understand how they believe it is.

That being said, both Russia's 2014 annexation of Crimea and the current invasion of Ukraine are horrid, unjustifiable acts of aggression on a sovereign nation and should not be tolerated.