White Crispa Anemone+Skimmer+Algae

[etesami]

Hi,
I had Planted tank for 10 years and I'm new to the saltwater hobby and I don't know much about saltwater.
I bought an anemone 4 days ago
After adding to the tank it's successfully attached to my live rock in been there in these 4 days
My question is how long is the period for feeding it and why its mouth is open always?
When I feeding it his mouth will be close but it has hole in middle I don't know if its mouth or etc.?
Second question is my Skimmer do light skimmate is it ok or should I get darker?
https://ibb.co/c9en4v
Third question is: why my substrate got heavy algae is it due to high phosphate? then is Seachem PhosGuard is good material for removing it?
what is your idea for removing algae of substrate? (Fish,Syphon or etc.)