If the anemone was purchased large and then slowely got smaller in your aquarium, it's most likely becuase lack of food. I first want to tell you that Micro Very isn't doing anything for your anemone. Anemones are carnavorous and will not consume small sized particles, which include both phytoplankton and zooplankton.
The anemones diet really depends on the species of anemone that you have. Some species will prefer krill, while others may feed on silversides. A picture or a detailed discription would help if you have no clue of what species your anemone is.
I also should get some more information on your tank setup, including your lighting, how old the tank, all water parimeters, all tank inhabitants, filtration, how long you've had the anemone, etc. The more information you provide, the better answer you'll receive. But, just from the vague information, it looks like your anemone is starving, which could be from lack of light or lack of food. Again, that's only from your discription. A more detailed driscription would lead to a more detailed answer.
By the way, welcome to saltwaterfish.com!
Take Care,
Graham