Not harmful in any way shape or form. Brown Algae is common in new set ups due to the constantly fluctuating levels of ammonia, just keep removing it and it should go in time, try reducing you lighting hours as well, no longer than 8 hours a day.
If you want it off then a new washing-up sponge (do make sure it's not one of the soap impregnated kind!!!), an old, clean toothbrush, or a piece of filter floss will remove it.
Don't scrub too hard, as some of thses ornaments/backgrounds have colour on them that is easily removed by hard scrubbing; and the brown algae will go on it's own eventually.