ph dropping is part of the nitrogen cycle. That means the bacteria is doing it's thing. A water change with higher pH water is necessary now to avoid the cycle stalling.
The prawn growing fur is also normal, as it is decomposing, and fungi are starting to break it down.
Let the biofilm grow as that will be part of the shrimp's diet once you add some in. If it's hindering your view, scrape it off the front glass only, but don't syphon it out. The dislodged biofilm from the front glass will settle somewhere else in the tank.