My LTA did a very similar thing for almost a month except mine ate every chance it got. Thats what I'd focus on, getting it to accept food. A part of that will be touching it to see if it "grabs" you. The sting feels more like velcro on your hand, kind of.
Anyway, my LTA had a swelled foot, but remained "alert" to stuff around him and held his tentacles up like yours does. Looking at your sand it looks rather coarse. LTAs bury their bodies into the sand for protection. your sand may be ebrasive/too coarse and that may cause some swelling. after a months time, the LTA that I have let itself drift its foot into an over hang of rock which it squeezed its foot into, then buried it and it attatched to the glass ot the bottom of my tank. I've tried a technique at my shop and told other to try it and has had very good results woth getting them to act normal and foot down. I just put their body/foot near such an overhang about soft sand, and almost all will foot down within a week, some decide to move after that, but many seem to be looking for positions like that. hope I've helped
J-Cal
PS: I'd like to add a question for everyone here. I've had my LTA for 8 months now and in the pask 2 weeks its almost doubled in size after no growth. I have started more frequent spot feedings, but My lights are getting older and will be replced on thursday. Is it spreading due to real growth, or is it trying to just get mroe light?