Yes, I should be able to
Consider it from a practical perspective: if something slips (is not being propelled), is it really moving under its own power? It does not have a flight speed, because it is not really "flying".
See the spell fall of feather:
Its descent is reduced to 60 feet per round until the spell ends.
Pen fall It is probably a good place to start, since this spell deals with the slow fall and lists a defined speed for your fall. Apart from this, the other source of fall speed that I can find is the XGTE rule (500 per round during the free fall). I think it's fair to decide that Manta Glide slows down to 60 & # 39; per round in a similar way to how it does fall of feather make.
In this way, it would only take one round to finish falling 50 &, during that round you could slip 100 & # 39; horizontally. Of course, it would take more than that if you fall off, for example, 120 feet (2 rounds, total horizontal movement of 240 & # 39 ;, 120 & # 39; horizontally per turn)
This would be a relatively quick fall, but it is probably not fast enough for other characters or enemies to have problems with your objective, if they can reach that height in the air. Your DM is free to make decisions about the details of how much it falls turn, but 60 & # 39; por round It's probably a good starting point.