> So it's entirely possible that fungi could survive on Mars while trees/moss/grass could not

Nope, there is no real chance to find fungi living at -50C to -70C, with no liquid water. There is no life on Earth that lives at such low temperatures, and there is no way for life as we know it to work without liquid water (or perhaps other liquid solvents, though that is highly speculative).

Also note, fungi in particular can't deal with extreme cold, the coldest I've read about any kinds of fungi living is ~0C. There are bacteria and other prokaryotes that survive at temperatures below that, but even then only at high pressures that allow liquid water to exist at very low temperatures - whereas Mars has extremely low pressure, much lower than anyone on the Earth's surface (Martian atmospheric pressure is comparable to Earth's upper atmosphere).

Whether life could survive is entirely orthogonal to my point, and sneaky life surviving that we can't even see is exactly what I said could still be on Mars. What can't be on Mars is life with stalks (it may not sound like much to a Earth-bound human in 2021, but that's actually quite sophisticated for life) that grows macroscopically over the course of just a few days. There'd be evidence of their metabolites, probably color and spectra changes like Earth's greenness as something uses sunlight, all kinds of things. Macroscopic plants are way too much to claim on the basis of a few photos. I mean, if nothing else, we ought to see lots of them.