Actually they are "soka gakkai"
"Namyo Horen Gekyo" qequals to "南無妙法蓮華經" or "pay homage to great dharma lotus sutra". The scripture they referred to is "lotus sutra". IF I remembered correctly, they only focus and chant certain chapters but not the whole sutra. You can get the original sutra translated in Chinese and english if you want to know the context of it.

btw, They are not equal to "Tendai buddhism"

How do you differentiate between "practice Buddhism" and "Dhamma Practice"? How do you define that?

Used to be Mahayana, now I follow mostly Theravada teachings.