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 are the synonyms of "glacial" (They all mean cold)
These are for "slow-moving", but none seem to actually imply the persistent slow movement of a glacier.
Best Answer
I have encountered creeping in the context of glaciers before.
creeping
Another suitable adjective is sluggish.