Ghost players (or shomething)

Well, I'm not even sure this is strictly related to Skyblock, as I haven't been playing for a while and maybe it's happening on the main servers as well. Here's the thing: I see ghosts.
Every once in a while, I've seen something that looks like a player hopping around me: I tried saying "hello" in local chat, but they don't answer (but this could just mean they're trolling).
They look like players, with a name above, but 1) they're partially transparent, 2) they fly and, most important, 3) they don't move like human players: they seem to mirror my movement, in an oddly temporized way.
I recall at least 3 episodes where this happened and in 2 cases the name and skin shown were different (can't recall the first one, wasn't paying too much attention).
I tried to punch one, and it seemed I could hit it, but... well, after hitting and hitting again I forgot I wasn't on main server and, well, I fell like an idiot into the void.
So: what's this shit? Am I being hallucinating? I tried to google this, but apparently "ghosts on skyblock" (and similar) are not the right keywords.
Thanks!

 

@TheWhiteFang123 and @12345shane are real players, so maybe they can confirm or contradict the fact they were actually around @OlympiansAreGods? I mean, if they say they were not (assuming this is not a huge prank), then it must be some sort of thing @soyuzpixel is talking about (but I wonder why it uses actual players' usernames)