brown diatom algae is indeed normal for any new tank. sometimes more intense lighting helps minimize it, or makes it clear up sooner.
Diatom algae is unlikely to kill a plant. You have a moderate amount of light...120watts over 55 gallons...just a smidge over 2watts per gallon. With only 2 bulbs, you'll have dim areas in the front and back, so plants won't do as well there.
And plants with high light demands won't do well in the tank at all, so unless you're upgrading the lighting, steer clear of anything that's medium/high light. Stick to low light and low/medium for a good success rate.
You're also at a light level where some injected CO2
will be beneficial.