Linda,
Welcome to TFP.... A Great resource for all pools owners needing insight to their sight glass issues.
You can do what I have done..
1. Turn glass bottle with plies...
2. Cuss a lot after it breaks into pieces..
3. Go to the store and buy a new one..
4. Install new sight glass..
Your best bet is to carefully remove it, clean it and the seat, and reinstall it.
But.. be prepared to get a new one just in case.
There may be a trick to its removal, but I am not aware of one..
Thanks for posting,
Jim R.