This is one for Leslie H over in Invertebrates. I've not seen one like this---but I wouldn't kill it or discourage it without knowing what it is---if the hammer is happy, it'll outgrow it many-fold. I'd be very interested to know what this is: common spaghetti worm is vaguely possible, since I've never seen the base of one, but I'm really curious about this one. At first I thought 'coco worm', which is sold as an ornamental for quite a few bucks, but do look into this with Leslie.