I have seen many times that this type of algae is ok in an octo tank. However, it is illegal in the USA for that purpose.

Attached is some info I found.

Despite bans on its possession in France, Spain, and Australia, this organism continues to be transported and sold by the aquarium trade; fearing its eventual introduction into US waters, over 100 prominent scientists petitioned the federal government in 1998 to ban the use of Caulerpa taxifolia in American aquaria, leading to its designation in 1999 as a prohibited species under the Federal Noxious Weed Act.

Are there any other types that would be a good "playground" for a bimac known to anyone?

there are hundreds of other species of caulerpa though... most shop should sell a selection of species.. maidens hair etc???

Halimeda, lettuce seaweed are just some of the many beautiful plants which should be legal.