Nothing we know about the world suggests anything at all, either way, about whether or not moss, or anything else beyond our own, individual selves, are experiential — and we only have that knowledge because we have direct access to our own experiences. This is the essence of the problem of Epistemic Asymmetry, according to which I can't even know if you are an experiential thing. Yes, of course, you look and act like one, but it's perfectly logically consistent with all of my experiences of you (and utterly, a priori impossible to disprove) that you don't. (Aside: if you have trouble with this notion, do some reading on zombies — and I don't mean the shuffling, groaning undead that like to eat brains. A web search on "Chalmers" and "zombie" should get you started.)

It might help in understanding the conclusion I've reached, to talk about how I got here. See, to my mind, no-one has yet satisfactorily answered Descartes' great question: How can I know that my experiences are at all reflective of an antecedent reality? His answer was to pull the biggest deus ex machina of all time, and asserted that reality is "real" because, well, God. That's a cop-out, if you ask me, and hand-wavy at best. So I'm left, philosophically speaking, stuck at solipsism. And, frankly, that sucks.

In the alternative, I have to look at the data available to me through my experiences, and try to draw some kind of meaningful conclusions from it. The first datum I examined is that all the people I interact with have an external appearance that's similar in form and nature to the thing I see when I look at myself (assuming we first posit that they actually exist, experientially or otherwise, and that my experiences aren't the product of some vast sim in which I'm trapped; if that's the case, it's game over already and there's not much point in playing). So maybe the other people have an inner, experiential nature, too. If so, what's special about people? Don't other animals, in keeping with their forms and natures, act in a way that suggests they're experiential, too? Maybe they are. So what's special about animals?

Follow that train long enough, of course, you end up in a bit of a reductio ad absurdum. You can cut the problem off by drawing some arbitrary line somewhere that says, "Things on this side of the line are experiential, and things on the other side aren't," but where can you meaningfully draw that line without access to the "inner state" of the things immediately on either side of your line? And what is it about those things on the one side that makes them special and privileged enough to be experiential, but not the others?

Following, instead, the principle of positing the fewest number of entities in my attempt to dig my way out of the solipsist hell that Descartes left me in, it seems least arbitrary and hand-wavy to posit that everything in the "external world" of which I have an experience might also somehow have an experience of its own. It's as arbitrary a place as any to draw the line, but unlike every other place, it doesn't require explaining why the line happens to fall there and not somewhere else. Moreover, it cleanly solves the question of how it is that meat can feel. It can, because everything can.

Please note, that's not remotely to suggest that the experience a moss has, let alone a rock, or a lone hydrogen atom, is qualitatively anything like the experience I have. How can they be? The nature of my experience is the product of absolutely squillions of different interactions of all the constituent particles, atoms, cells, organs, systems and structures that make up the vessel in which I have my experiences, each having their own individual experiences of being that part of the vessel. And it's likely some of those interactions are more directly relevant to the nature of my experience than others. My femur, for example, probably has less bearing on what it feels like to be me than my spine, but it's absolutely a factor in the complex interplay that gives rise to the whole thing.

But again, at the end of all that searching — and this literally took me years of pondering, mulling, barking up the wrong tree, false starts, and starting over from first principles — it's the least arbitrary answer I can find, it posits the least number of things in the universe (only one: experiential stuff, otherwise known as "matter"), and seems to me to neatly address some absolutely huge, and eternally open questions.

So, yeah. It's only a theory, but it's the best one I've got, and it beats the hell out of solipsism.

EDITS: there were many.