If your marimo ball can "unroll" and become like a flat patch, then its not a natural marimo ball. Its a hand-made version whereby pieces were torn from an existing marimo ball, then hand-rolled into a ball, hence it has seams and can unroll. Not much you can do other than to hand roll it back into a ball shape each time.
As for the brownish patch, thats usually due to lack of light going to that area (especially if its sitting in one position for prolonged periods of time). Just rotate it in the tank so that the brown patch can face the light. It should recover and become green again in time.
To ensure consistent green color on marimo balls, regularly rotate them so that all sides get light. In nature, marimo balls float, move and roll around in the water current (which is how they form their round ball shape).
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