Algae really harmful just not the best to look at. if you have had the tank for 18 days it's probably not fully cycled yet so that is kinda normal. I have never tried this so I'm being kind of a hypocrite but if you leave it alone and have good water parameters it should go away on it's own.
I'm no expert but I have successfully kept a variety of planted freshwater tanks for about 15 yrs on and off and I have never done the whole break down and start over stuff that people always seem to suggest. if you didnt use cycled water from an existing tank or an already established filter it's probably just algae from your levels being a little off, I wouldnt worry about it too much if you dont have a pleco I would get one or some snails
Our old tank pump/filter went out so we sorta scrambled and used the opertunity to pup grade to a ten gallon and get it up and running for our plants and betta. I did use the old sand and seded the new filter with the old one. but it came from a smaller tank I went ahead and wiped it off just because it buged me. we havent added any more live stock to the tank yet I thought it would be a good idea to have it running a little longer but the girls are getting anxious to get new friends glad to hear it will probably go away on its own.