I just used the search function to look up references to white tongue here in the forums. It appears you are not the only one to notice this effect when beginning Keto. At least a few people experience this and it resolves after awhile. Might be related to your gut flora changing (some dying off and new flora flourishing) since they aren’t getting what they need anymore.
You might want to search yourself and read a few more of the posts referencing white tongue to see what other possibilities other people think might be going on.
Good luck!