I've never found a proper answer to this. The implications of the FCOM is that the alert height will not result in a change in landing capability if there is a failure of the redundant part of the autoland system. Ie. CAT 3 DUAL will remain so. This actually occurs at 200ft concurrent with the autoland light arming.
However airbus have assigned this number as 100ft for the single aisle fleet.
What happens between 200 and 100ft is no mans land. Presumably nothing as youd not know anything about it. The implied purpose of alert height and it's protections have already occured 100ft earlier at 200ft.
In short, to answer your question, go around for the red autoland light.