I dunno... I've never seen a bubble tip with a foot that was a different color from its disk/tentacles. Perhaps it's bleached? But does bleaching only affect the tentacles? The tentacles certainly make it look like a bubble tip, but I've never seen coloring like that on a bta. Any pics of it during the day, under a full spectrum bulb? The first one looks like it was shot at night, wtih the flash, the second one looks like it's under actinics.
I'm wondering if it's a sebae.