clipped

My gut reaction is that the lower seam/joint is sealed with some kind of
caulking, silicon or otherwise.

How to remove the old caulk, and what to relace it with.

My instant reaction last might was a utility knife and a lot of patient
scraping and cutting. Maybe also a small, small flat bladed screw driver to
scrape the old caulk out with?

As to recaulking, a silicon caulk, either clear or white? Alex is one brand
I know of, and there are several versions of GE brand silicon caulks. Any
suggestions?

A razor-blade scraper is my favorite tool. Screwdriver might chip tile.
Utility knife point is good for gap between pan and tile. What is pan
made of? Not tiled? I would take pains to keep the stall dry for about
a week in case (probable) that water has seeped into the gap. Wipe with
pure bleach and let dry just prior to re-caulking with silicone caulk.
Painters tape is nice for keeping caulk where you want it - be sure it
is dry and smoosh it down into grout lines so's the caulk doesn't go
under the tape. Take tape off right away.