Idea to seal (prevent water) in the gap between marble floor and fixed shower glass panel
Clear silicone is very very very easy to take off marble and glass.   There is no residue after hitting it with a glass scraper and some mineral spirits - a big shower is maybe 15 minutes.   

For application on something like 1/4" I would put a first coat down that should fill most of the gap.   Then come by about 1-3 hours later (depending on air flow and temperature of room) - you want the first coat to harden a little but not be set - and put a finishing coat that looks good and fills the rest of the gap.   Do it right. 

(There are also some edging strips that you could use.   You would effectively have to use glue - silicone - to install them and they would be a PITA to remove.  But the big issue is this shouldn't have been installed with a 1/4" gap)