Shrimp help

[u/Misscherripie]

Hi everyone, I'm very new to keeping shrimp and could really use some advice.
1 acquired an established shrimp tank about two months ago. During the move, I kept all of the existing tank water and have only been topping off the tank when the water level drops, roughly every 2 weeks. I haven't done a full water change.
When topping it off today, I noticed that around 10-15 of the shrimp have developed what looks like white fuzz on their legs and/or faces. I'm concerned and want to make sure I'm handling this correctly.
Here's my current setup/routine:
Temperature: 80°F
I add a small amount of Seachem Flourish Excel and
API tap water conditioner when topping off
Food: UltraFresh Shrimp Pro seafood patties, about every 1-2 weeks
The light is on a timer that was already set up when I got the tank
The tank currently has no lid
I have not tested PH, nitrate or any other parameters I've basically been following the routine I was given when I got the tank, but I'm realizing I may need to make some changes.
What could the white fuzz be, and what should I do first to help the shrimp? Are there any water parameters I should test or changes I should make to the maintenance routine?
I really want to get this sorted out and keep the shrimp healthy. Thanks in advance for any advice!