Hello,
There can be many reasons for the condition you describe. One of the most common is protection from drying and/or UV when exposed to air. Acropora at low tide can produce great quantities of a slimy mucous. Every water change on my home tank results in this happening.
Another cause is a reaction to other corals either near or far away. Many corals produce toxic compounds that will trigger a reaction as you describe.
The addition of additives will also cause some corals to temporarily slime up.
For Acropora corals extreme poly extension is the normal observation of it when feeding whereas sliming is most often associated with irritation.
HTH,
Kevin