kinda inevitable with coco, since you don't want to let the top 1" dry before watering like soil.
algea requires light.. it can't go into substrate.. you could remove the perlite on top, if you want... it serves no purpose there anyway.
i've left it befoer and had no issues. i mostly fill reservoir, thouhg.. .top-fed may get stripped of a minor amount of nutrients, maybe?
You can cover the top of the pot -- paper plates or something fancier that is made for the function.. blocking light will stop the growth in future. pain in the a@ to water, if not automated. i quit bothering, lol.