If they aren't doing anything, I'd take them out as they may be rotting. It could be bulblets from a non-aquatic plant.
If any of them feel firm (not mushy and rotted) you could do a cold treatment (to simulate dormant conditions) for about 4 weeks in your fridge. After that, plant half in a pot or your yard and the other half in your aqaurium. See which ones start sprouting roots after a week or two and you may get an idea of what it is. (Of coarse, this may not work either.)