Here's the description in the wiki:
Hyper-Dimensional Anchor Planet, Effect (Permanent) Permanently boosts a planet's passive cloaking by 90 when used. Cannot be used on Oceanic Worlds.

Now I'm pretty sure I already know the answer to my question but I just want to verify. What happens when you use hyper dimensional anchors on an Icy planet that then subsequently has a Mass Surface Melter used on it (thus turning it into an oceanic planet)?

My gut says the passive cloak is retained as the artifact was used on a planet that wasn't an oceanic at the time. The same principle can be seen when you turn an icy into an exotic and it happens to have a Permafrost Network on it. The bonuses for the permafrost are retained even though the planet is no longer icy.

But that said, I would still like some verifiation on this.

Yep, passive stats stay. Passive stats are passive stats. Unless a planet artifact says "decreases the planet's [passive] cloak by XXX", the passive cloak won't change