algae?

hay im growing a few plants from seed using rockwool as my medium and the rockwool cubes have now got a algae layer on the top should i be worried why is it there? how will it affect my plant? thanks for the help in advance

they sell squares of white plastic size to fit

yes i know but im asking if it will harm my plants

put the palstic on it and then you will not have to worry

im not asking that