In the 2nd to last paragraph of this chapter, the narrator gives a clue why he thinks Mars is red. What does he think of it?

I think this has something to do with the red weed he discovers

For a space I could not understand it, and then I knew that it must be the red weed from which this faint irradiation proceeded. With that realisation my dormant sense of wonder, my sense of the proportion of things, awoke again. I glanced from that to Mars, red and clear, glowing high in the west, and then gazed long and earnestly at the darkness of Hampstead and Highgate.