Light colored substrate seems like it might be a key to horizontal growth with HM. I had it for years with 217 w/gal. It grew straight up with FLuorite. I gave some to another guy who has a LOT less light, but is running a really light colored substrate almost like the old standard tank gravel and it grows out in runners all over.
Eheim Pimp #204.
58 gal planted Oceanic
Red Turquiose Discus, Japonica shrimp, assorted Blue Eyed Rainbows, Tetras, Hatchets, Danios, Rasboras
10 gal low tech planted Celestial Pearl Danio (Celestichthys margaritatus) & Emerald Dwarf Rasbora (Microrasbora erythromicron)