My suggestion is leave it if you can. Why draw the eye up to vent? Your plant in front of it is probably rendering it inefficient for your heating / cooling? So if your remove the plant, you remove a reason for the eye to travel upwards. It is hard to say without seeing the big picture and how "in your face" this shelf is. But based on the information available, I do feel less will be more effective.

I would decorate with your collectibles in groups of three or five. Large items, not too small. Vary the heights and place like items together. Paint the vent the same color as the wall. Vent will tend to disappear. Keep a lot of space between the vignettes. Do not put anything in front of the vent. By having the noticeable display items, the eye will tend to ignore the vent.

A picture of the whole wall would help us to see how that niche and shelf fit into the space as a whole, and how/whether it should be decorated at all, as well as, perhaps, why you want aqua there and how it ties in. The easiest way to get aqua into the space would be to paint that little triangle of wall -- and the vent -- in the colour you want -- but it could, probably would, look awful. One can't tell.