It's a stem plant...you can grow in just floating in the tank as long as there are enough nutrients in the water. Looks like the bottom is "melting", not the tops so that indicates the light isn't strong enough. Yes, 20w is 4wpg in a 5gal tank, but 20watts isn't much light even if it were over 1gal.
You aren't overdosing excel are you? Although I don't recall HM being sensitive to it (is it...isn't it? :wall
. As long as there is a majority of green, it can be saved. Do you have fish in the tank or just shrimp? Feed often/change water often?
If I'm trying to save a stem plant, I let it float so it gets all the light it can. If you have some nutrients in the water, it will show new growth within the week.
I don't know about emmersed grown HM...just haven't heard of it. I don't claim to know everything either so I won't say it isn't a possible scenario