I believe he doesn't think his stories are filmable. If this has anything to do with his crankiness I have no idea.
I know a lot of stories are not made for Hollywood adaptation, and require massive rearrangement to become movies.
For example, take the classic The Strange case of Dr. Jekyll & Mr. Hyde
. The book has no love intrests. Most of the action takes place via flashback. Mr. Hyde is actually barely in the book at all. The story ENDS IN A FLASHBACK. The main character is actually Utterson, who is just an observer, and thusly useless unless you were to keep the mystery aspect. and of course, everyone knows what's going on so the mystery aspect is also useless. Turning that into a Hollywood formula requires that the entire storyline be essentially thrown out and replaced, even though the book is excellent as a book
It's possible Moore is simply unhappy with Hollywood commandeering his work. A lot of artists are possessive of their ceations, which I can sympathize with. I guess Hollywood clinging to his coattails annoys him. Others write their stories specifically so they will be easy to adapt.