Acid rain is no longer the problem it once was because of laws that have been passed since the 1980s. So I guess in that sense it is not an invisible threat because it is not such a threat anymore.
However, in cases where it is a threat, it is an invisible one. Back when acid rain was a problem, it is not as if the rain fell and you could see or feel that it was acid rain -- it didn't dissolve things it hit right before your eyes. Instead, it seemed like normal rain, but gradually it did dissolve things and it did contaminate lakes and such.
So when acid rain is a threat it is invisible because people are not able to know when the rain is acidic or see the damage it causes in "real time."