The simple answer is yes

The nuanced answer is yes, but this person isn't really the problem. They are still profiting, ultimately, off the labor of another just by owning "capital" (quotes because a personal home isn't really capital, but it's sort of transformed into this specific case... it's weird). However in terms of scale and intent, it's different.

Scale obviously if you rent one or two rooms that's the limitation of exploitation you can participate in unless you become a full fledged capitalist and buy more homes to rent.

And intent is obviously a gray area, but looking at it in the least cynical way, the owner in this case might be struggling for whatever reason, electricity rates are high as fuck, inflation generally is up, and they might legitimately need say $400/month more income to just stay in the home. In this case that would be cheap as fuck to most people to then be able to live in a full house with a private room. This intent, in my view, is as ethical as possible for this situation. If the owner is otherwise doing fine and just wants to pocket $400/month, well, that's essentially just being a normal landlord, although still a little different since the home wasn't originally purchased with the sole purpose of profiting from renting it... so there's a lot to analyze there and my end thought is basically "this person isn't worth the time to care about even if their intentions are the worst possible case." Basically because there are much larger fish to fry. That doesn't mean I have to support it, but also hyper-focusing on some dude/family renting one room out is... not a good use of time, imo.

This still benefits the homeowner of course because they, and they alone, gain equity in the home as they pay off the mortgage more and more. If a tenant lives there for 5 years, $400/month, and moves out, the homeowner has effectively stolen $24000 from the tenant. Obviously some of that went to, perhaps, paint for walls, upkeep from increased wear and tear, but let's be honest it wasn't $24000 worth unless the tenant collapsed the fucking roof. This is why it's exploitation and the only real way around this is some sort of legal agreement that the tenant will receive their full portion of the equity if they move out. Maybe if the owner is elderly putting them in the will would be a way to do it. There's a bunch of scenarios, but if the tenant just lives there, pays, and moves one day, the owner directly benefited from someone's labor, so, still exploitation. Although this comes back to my above answer of "this is so small that it's hardly worth focusing on."

Morally speaking I don't think it matters in and of itself. There's a power imbalance that needs to be critically engaged with so it doesn't become a problem, and there's a small to medium influence on your material interests that may color your perspective on things if you aren't ideologically disciplined.

Treating people well and being nice and trying not to exploit people is basic, individual morality, trying to be good.

Leftism however is about trying to understand and change society. It isn't about lifestylism or poverty or personal purity.

Categorically speaking I wouldn't consider you a landlord. Your material interests likely don't align with the landlord class, assuming you still have to sell your labor to survive. Of course you (or others in your material position) might adopt an aspirational consciousness that aligns with landlord ideology-- I've even seen that from people with no private property whatsoever, nothing at all with which to rentseek. Temporarily embarrassed millionaire types.

rather than rent, sell them equity in the house, or if you're subletting, team up on the actual landlord and they can have their own place.