Can I use icing sugar instead of caster sugar? Are they the same thing?
Icing sugar is much finer than caster sugar.  However this way round you can probably get away with it ("probably" because without the exact recipe it's hard to tell).  I have recipes for butter biscuits that differ only in one using caster sugar and the other using icing sugar, for example, and cake recipes that use icing sugar. Icing sugar clumps badly so can be less convenient, plus it used to be more expensive though these days there's nothing in it.

You must work by weight when substituting between coarse and fine sugar, as they pack very differently into volume measures.  If your recipe works by volume, you shoudl go out and get the right sugar.

What you can't do is use caster instead of icing sugar in icing, as there's not enough liquid for it to dissolve.