If you've never seen it on the surface then it probably isn't either of those. They are both very aggressive and usually start shallower from my understanding and grow VERY fast.You should have noticed them before now (unless it's somehow just getting started). I did some refreshing on both last night. Hydrilla actually can grow even deeper.

Only way to be sure is to try to pull up some more and post a pic so we can see it.

.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!