well there r 2 other possibilities but youd have to describe to me if they had these, does it have a fan shaped top? or any tentacles? because one options could be a species of a hydro, which is basically a clear tube with tentacles that paralyze food then it inverts itself to eat it. thats not very common in saltwater because there are only 2 species most are brown or clear. and alot smaller, but it might be if it has a fan shaped top and if it does dangle some food near it if you see any tentacles attatch to it then it very well could be, also this is what happened to me, when i purchased my tank some of my live rock had mysterious soft polyps on it kinda like that, then they dissapeared and a week later i saw very very small forms of jelly fish in my tank they swam to the top and slowly drifted down collecting food, but since my tank was new and i had growing nitrates they dissapeared for good, unfortunate too they were amazing, so this is very rare because this stage of jellyfish is usually too hard to establish itself, but if it is then thatd be awsome, it could be its earlier form, oh and a 3rd thing it could also be a tunicate or sea squirt but it doesnt really look it from those pics, you wont find much on hydros but definitley look at tunicates, sea squirts and polyp stages of jellyfish to indentify it, i highly doubt it is jelly fish it looks too tall but there are alot of smaller species out there... for polyp form anywheys .. Oh and behind it lies an aptasia if you want this thing to live or develop for a desried period of time get rid of that aptasia because the sting might kill it. if u didnt kno what it is already alright well good luck can u get any other pics? i hope all that didnt confuse u its just jellies (doubtful) hydro ( more common than jellies) or the other 2 things but ya eh good luck on identifying it...