What To Do With Brown Stuff?

Hi, reporting in again, I've now had 6 Harlies for 5 days and they're doing OK. I've taken to calling them the babies 
However the tank is now getting covered in brown stuff  , I'm assuming it's brown algae but since I've never seen brown algae I'm not sure, either way if I scrape it off the glass do I need to scoop it out as well, not sure if it's Ok to float around and end up in the filter?
Two, plants have given up the ghost and I've now removed them but the others are doing ok apart from some of the brown stuff covering some of the leaves.
Had a minor panic  tonight as the ammonia seems to have risen, but I've done a water change and it seems to be ok now, suppose it might be the dying plants, as I only noticed them tonight and maybe the newbie error of over feeding, although I've been trying not too!
Thinking if I test the water and it's ok over the next couple of days and get rid of the brown stuff, I could maybe get my next 6 harlies on Monday, and a couple of replacement plants, what do think?
Oh and final question the harlies swim quite fast when the filter is on full and seem much more chilled when about half power, should I keep it like this or does the filter need to stay up high, especially if the water is going to be a bit dodgy?
As always any advice appreciated? 

Thank you, good advice as always👍
The tank is 240 litres, filter is a fluval 306, tech spec has this following, not sure which one is flow?
Pump output US Gal /L/Hr = 303/1150
Filtration volume US Qt. / L = 3.26/3.1
Filtration circulation US Gal / L/Hr = 206/780
Not sure which one it is and there is also the following
Mechanical area
Biological volume
Filtration volume
Max water column height
Do you need any of these?

Brill, I'll change that now and follow your advice re water testing, patience is the name of the game!

No doubt talk soon.