To tell you the truth, I don't know of any slow growing photosynthetic soft coral. But the good thing about most soft corals is they are easily pruned by cutting them. Sinularia (finger leathers, cabbage, etc.), Sarcophyton (leather, umbrella), Lobophytum (finger leather, devil's finger), Cladiella (colt), and Capnella (kenyan tree, finger leather) are a few genera that are hardy and easily pruned.