I'd say, reduce the cost to two, but have the ability have a cost. To reduce confusion and make it easier to use, Manta Ray can protect one monster for one turn. The ability cost being 2. You could raise the health to 8 or 9 and reduce damage to 1.
That ruins the point of it, it's supposed to be a constant Cloak (and you don't see Cloak having an ability cost). Also, it's already likely UP as it is, since it costs the same as Cloak and yet protects significantly less things, even though it does last much longer. But considering that RT, Lobo, LS, etc. all can counter it (not to mention that AoE CC can remove as well such as RoF) and that CC is much more popular than PC, I think it doesn't need a nerf.
Also, it's not a particularly confusing card. What is there that's confusing about it? The only remotely questionable thing is the interaction of Mantas with each other, but there are plenty of in game cards that are much more complex (I'd like to say that Antimatter, among others, ultimately has many more situations where you wouldn't know what would happen without testing).
Some art if you want it
And I'd also lower the ATK, though not necesarilly to 1. 2 would be reasonable I think
Thank you, I shall add once I get computer access.
Also, any reason for attack nerf? Its cost:attack ratio isn't too spectacular anyways.