Andorian ice fever probably.

It’s covered a bit in this article: Ex-Astris-Scientia.org - Star Trek Races with Changing Faces

My understanding of the above is it’s basically likely to be a mistake. It’s suspected they used bluescreening (we tend to use greenscreening these days with digital video, but blue was common back then with film) to overlay the different versions of Lal - and may have been worried about the blue Andorian getting mixed up with the background and becoming transparent - so they changed the colour of the person being filmed, then tried to correct it in post-production… and failed. This hasn’t been officially confirmed anywhere, but it seems a reasonable explanation.

An in-universe explanation suggests that green Andorians are Andorian/Aenar hybrids - but the holographic Lal here is not so much green-skinned as lit up with a green glowing light.