Just noticed this growing up from my RTL. Is it a flower?
20 gallon long
Nat geo 30.5 inch led light(7 hours per day)
seachem flourite substrate
seachem line of additives (6 days a week using the tlar method)supplimented with guppy dung.
co2 ista 1 litre and fluval diffuser 1bbl/3sec
filtered with fluval 206
50 watt heater 25 degrees c
Looks like what my Crinum Natans did about this time last year. Started out as a thick round stem similar to what you have there and quickly shot up to the top of the tank. I opened the tank cover a bit and it went through the opening several more inches and within a couple of days I had several blooms that lasted a week or so.