Help with mushrooms in shower

Moved into this house about two years ago. This shower was built in 2016 as an addition to the house. But now we had some mushrooms growing in the shower edges: https://i.imgur.com/cfy54HT.jpg

Cleaned, disinfected, and dried the shower completely. I avoided using this shower and haven't seen anything new grow in weeks.

My current plan is to remove the old grout, clean a lot more, and apply silicone caulk around the edges of the shower instead of grout.

Never done anything like this before so is there anything else I should consider? Not sure if the mushrooms mean a bigger issue exists and if caulking the edges would be enough to prevent moisture there.

A few more images
https://i.imgur.com/b94E4BW.jpg
https://i.imgur.com/a2FBZuO.jpg
https://i.imgur.com/d2B03Ex.jpg

Edit: Some images from below the shower area
https://i.imgur.com/HWbIfqr.jpg
https://i.imgur.com/JSCyxnG.jpg
https://i.imgur.com/X3FW9Iv.jpg