So, yesterday I got some opae ula for the first time, so ofcourse it comes with a couple of questions since it's the first time I'm setting up a tank as well

I've been browsing the forum a couple days and probably overlooked some of the already asked questions, so I'm sorry if some questions have already been answered before.
I set up a 30L tank in my new studio as mentioned in my previous post about the luminous sand.
It's been cycling for a good 4 weeks now and some oily looking film has been covering the top of the water ( biofilm?)
When I tried to add some cultured rocks to help the tank cycle faster I noticed a white slimy substance around one of the decoration pieces that had been in the tank for couple weeks.
I removed the decoration piece from the tank and put the cultured rocks instead. ( more reasons for me being scared to put the shrimps in there

For now I'm keeping the shrimps and snails in a plastic container since I'm not living in my studio yet.
I don't want to just put them in a tank I won't be seeing every day to see what's happening .
I put a couple cultured rocks in there too, and they've been happily swimming around and grazing the rocks.
I kept a close eye on the shrimps , scared I might lose them all in one night.
I took a couple shrimps out of the water and kept them in a seperate bowl just in case some shrimpocalypse would happen

I went to check on them at midnight and was greeted with red colored shrimps instead of pale colored ones.
I figured the red color means everything is alright so I put the seperated shrimps back into the container.
Sadly to say, the next day I did find a couple dead shrimps.
I'm not sure what dead shrimps look like, but they still have the bright red color, they're just lying on their side and aren't moving :/
So here a couple questions:
1. The white slimey substance around the decoration piece, do you think it's harmful to the shrimps? Any idea what it is?
I put a couple snails in the tank to check how they'd be doing. I haven't been able to check up on them yet but I hope they're alright.
2. Should I leave the dead shrimps be? It's a pretty sad sight but it doesn't seem the shrimps are too bothered by it. ... From time to time the dead shrimps do move from place making me think more shrimps have died

Thanks for taking time to read my questions
