The risk is essentially negligible. The environments are too extremely different for either one to directly invade the other.

? Invasive species like humans can do a lot of physical damage to the environment that other life depends on.

Sure, most humans cannot infect gorillas but they sure as hell can take away their habitat.

We're talking about microbes though.

Are we? I would have likened those species under Antarctica ice to ~plants (well, sponges are technically considered animals). Invasive starfish species and algae are a thing, for instance.

Some invasive species have adapted to sustain themselves with little light or nutriments. When placed in richer environments, they can reproduce really quickly, outnumber other species and drive them out by competing for resources.

It also depends if it was "closed" for how long, how "closed" it is, and the nature of the contamination.

The most dangerous potential contamination would seem to be from similar modern ecosystems in other parts of the world.