You could sand down parts of it to make it more rounded and natural looking. Since it's so soft it probably wouldn't take much time at all to change the shape to make it less square.
One thing that would probably really help would be to get a marimo ball and cover the wood with it. It would help soften the shape of it without being obtrusive, and it doesn't hurt the marimo at all. Simply feel around the ball for a soft spot, then slowly peel the ball open and tie the moss onto the wood. It will attach itself fairly quickly and as it spreads it will give the wood a much more curved organic look.
This site has an example: http://plantedaquariumscentral.com/M...l-dwmarimo.htm
You could also break it into a smaller pieces and make a betta lean-to out of it. :)