When we're talking about potential alien lifeforms, assumptions like "fungus eat plants" need to be loosely held. This may not be fungi as we know it at all, but rather an alien analogue that fills a similar role despite having completely different origins. On earth, we know that evolutionary forces can "recreate" similar organisms independently. The same could be true between earth and alien worlds. Ichthyosaurs and Dolphins have a lot in common but aren't related [by earth standards]. Maybe there are comparable similarities between life on different worlds.

Ok, makes sense - if there really is some life form on Mars then it's rather a humble mushroom than a xenomorph.

Not quite true, there are some rock-eating fungi. https://www.sciencemag.org/news/2016/03/iron-eating-fungus-d...

Also fungi that "feed" on ionizing radiation. https://en.wikipedia.org/wiki/Radiotrophic_fungus

interesting - didn't know this! Well I think we can confirm there are plenty of rocks on Mars.

These are all statements that are true of earth fungi, but we have no idea what is possible with a hypothetical martian "fungi".