Yeah, reading silly manga with slavery in it that's portrayed in a silly idealized way is essentially harmless. You can read fiction with bad stuff in it, even where that bad stuff is good, and have it be fine. But I think it is worth pushing on the morality of fictional scenarios sometimes, just to check you're not unconsciously picking up bad beliefs; it can't really hurt. Like if you were starting to get, I dunno, that being enslaved isn't bad so long as you have good circumstances, even if you don't have any choice in those circumstances, or something.
The thing that makes me cant my head is stuff that doesn't commit to "Our protagonist is okay with owning people as property, you know, like a bad person" or "Our protagonist would like to manumit his slaves, but can't, and being good to them is the best option available" or "The slave enjoys being a slave and in right mind to consent so protagonist don't worry about it". There's a lot of variety in there: people who are explicitly sadistic, people who think it's wrong but put selfishness above that, people where slavery is normal to them and they don't question it; settings where setting a slave free is not legal or a certain class of person would be free for anyone to re-enslave after, settings where without protection they'll get killed, slavery that's magically enforced and the protagonist can't break that; extreme submissives who sold themselves into it, stuff where they just got used to it, stuff where eventually they come to a consensual relationship and like the slavery angle for kinky reasons or a sense of connection.
Which I don't mean to say these make it "morally permissible". There are strong reasons to at least argue that people under the extreme duress of slavery can't meaningfully give consent to sex with their masters, or that you can't meaningfully trust that a slave is genuinely consenting and not just going along out of fear of reprisal or trying to keep the benefits that come with keeping you happy, that's a whole thing. One of the cases is the protagonist is just fucking Evil McSlaver, after all. What I mean is it's weird to look at manga where the slavery is there for the obvious kink/fantasy appeal (which again is fine) but the protagonist isn't cool with slavery (to be a good person) and there's just no answer within the story to "Hey, why haven't you freed them?" It's common enough that it feels like it could communicate some weird ideas to somebody as people contort things to resolve that. But I really don't think that's a huge problem.
I also feel like going too hard the other way and denying the conceptual possibility of a good relationship between an enslaved person and someone with power over them is... not great? Like it's super wrong, people can imagine that easily, and then it will undercut the whole point of trying to say that. Trying to point at history and say "There are no shades of grey with this" will always fail, shit is just too damn complicated. The point should more be, while it's easy to come up with a scenario that could be in a shade of grey, if you look at the overall situation—all the pressures, all the uncertainties, all the social factors—it makes those seem less likely. But people also like to treat slavery like it's monolithic and specifically like American chattel slavery. Trying to crush it all down to one thing, to better condemn it, is both wrong and counterproductive both to understanding the past (and present, unfortunately!) and combatting attempts to romanticize or rehabilitate the idea of slavery. But obviously, the whole... being enslaved thing, in and of itself, is a major bad thing, whatever other specifics.
Anyways, gorgon cute, I like her. It is a bit odd how common Medusa as a generic term got, though. Was it Dungeons & Dragons? The character getting more exposure than the type of her and two sisters who don't do much else? Does it just sound better to people? I wonder if one of these days something is going to start calling them Euryales or Sthenos, for Extra Variety. (Euryalai? Sthena?)