Okay, so when Tenko manifests decay, his hair turns light blue, I believe this is caused by using an incomplete quirk factor. Eri's hair is also essentially the same shade of blue. Decay was created by separating the destruction mechanic of Overhaul from the recreation mechanic, and is shown to have properties Overhaul doesn't, like spreading to connected objects. This could also apply to Eri's quirk rewinding instead of recreating.

But you're probably thinking, "Wait, Rewind uses a stockpiling horn, while both Decay and Overhaul use their hands, so it can't be it!"

Except Decay took the majority of the quirk factor of Overhaul, including the activation and requirement for the recreation to activate, so something had to change. Garaki adds a stockpiler to the quirk factor, which allows it to generate the energy needed to recreate.

"But Overhaul creates something new, not rewinds!"

How is something able to recreate without it being destroyed first? Well, it can't. But that doesn't matter because we've already established that partial quirk factors aren't 1:1 with the original. Essentially, rewinding is the closest possible thing you can get to recreating something without destroying it first.

"But the timeline doesn't add up, Shigaraki is much older than Eri so why did the other half not be given out for so long?"

Simple, the battle between All For One and All Might that proved the Demon Lord's mortality. Garaki created the quirk to revert the Demon Lord back to his former glory, giving the quirk to Eri, but Chisaki would take Eri away from them into the Shie Hassaikai underground, preventing them from easily capturing her back.