He does not like being scrutinized that carefully...

They get a little skittish about things that look odd to them and he is probably just being a normal betta. It means he is not feeling bad and that is a blessing.
I think you can used one of the other preparations if you would rather. The big issue is what to do with Grim while you are doing it as you cannot transfer the filter and heater with him. He will need to be in a heated tank or bucket though now he has gotten used to warm water, he will get finrot or one of the cool water diseases even more easily. I am so sorry to have to give you all this bad news in one day. I would rather not most days tell people things that are going to amount to work for them, but I know you all want to do the best for your fish.
There are algaes like hair algae that can cause the type of thing you are talking about. If you want a comprehensive artice on algae...here is one. The hair algae is usually dark in color though.
http://www.plantgeek.net/article_viewer.php?id=9
Please do keep us informed on how things are going. I do worry about all the little fish on this board and the owners too. Take care.
Rose