"I think "Wild Justice" would fit him very well. You have the word play on wild and wilderness, as he literally sprouts plants from his body, it also works as a nod to how a bull is wild in nature. His mix of Aokiji and Kizaru is addressed since referring to him as wild much like a bull alludes to his unpredictability and aggression; you have Sakazuki telling him to "not do anything unnecessary" which could also hint at that same hair trigger nature. The word wild encompasses both aspects, the animal and the plant, but also refers to them in the feral and undomesticated sense, like a plant that grows spontaneously as well as being self sustaining would be referred to as wild, much like Greenbull who hasn't eaten for 3 years and been literally self sustaining through photosynthesis.
To have wild justice is to follow the law of the jungle, which is the survival of the fittest, weeding out the weak through force or the threat of it, similar to Akainu, which could be why he was accepted as an Admiral in the first place, maybe Akainu took a liking to his way of justice, saw he could be a matador in a sense and direct this wild beast as he pleases, as matadors are referred to as "bull tamers", they also use a red cape (Akainus signature color). But the color Green also contrasts aggression, it's more representative of rest and tranquility, as we see with Ryo's very laid back and plant like nature, which only serves to contrast his dime stop like rage, as he sees red when someone crosses the line. "
I wasn't too far off. :cheers: