I didn't know they are an actual couple in the anime. I heard that theres a bit on yuri on it, I thought it was just some subtext thing or a one-sided-for-the-sake-of-comedy-thing... I never knew, haha.
They're pretty clearly in a relationship in the anime. My personal favourites are Michi's comment in the Super S movie about "Becoming an adult makes it more fun" which sets Haruka into a coughing fit, and the scene in Stars where Haruka's knocked off a roof and Michiru catches her. Michi jokes about Haruka eating too many sweets, and the reply is "Save the scolding for bed." XD