Question: I usually use "glacial" to denote something as very slow-moving, but this time I am actually talking about a glacier so it can't be "the glacial glacier".

[These](http://thesaurus.com/browse/glacial) are the synonyms of "glacial" (They all mean cold)

[These](http://thesaurus.com/browse/slow-moving) are for "slow-moving", but none seem to actually imply the persistent slow movement of a glacier.

Answer: 
I have encountered ***creeping*** in the context of glaciers before.

[***creeping***](http://www.merriam-webster.com/dictionary/creeping)

> 
> advancing or developing gradually.
> 
> 
> moving very slowly at a steady pace.
> 
> 
> 

Another suitable adjective is ***sluggish***.
thanks to this question I was able to discover a new word! Try **[testudineous](https://en.wiktionary.org/wiki/testudineous)**:

> 
> **Alternative forms** [testudinous](https://en.wiktionary.org/wiki/testudinous#English)
> 
> 
> *Adjective* (comparative more ***testudineous***, superlative most ***testudineous***)
> 
> 
> * Characteristic of a tortoise, or the shell of a tortoise
> * As slow as a tortoise
> 
> 
>