Now you know there's something wrong when I'm actually giving advice on topic and Biff starts talking about boobs :12: what the hell happened? :lol: ....Don't get me wrong, I like it, I'm just confused by it.
...back to topic, YES, you will need a bigger shell, and when I was looking for one, no one had one, not my LFS's, not Petco/Petsmart, none of those places. And then when you go online, it's tough to judge how big they actually are. A bunch of people told me to go to Michael's or Joanne's (Craft Stores) couldn't find them there either. So I ended up going to a gift shop by the beach that specialized in decorative shells (later found another even closer than the one I went to :frustrat: ) and got a few different sizes for Hermie to pick through. Luckily he picked my favorite of the colorations, which I was worried was going to be to big especially how he couldn't move it at first, only drag it. Within a month or two, he was back to climbing the rocks/plumbing and doing all his other circus tricks (i.e. hand/claw stands, cart wheels, etc :lol: ) These guys are definitely entertaining. If you need some shells let me know, I can send you a few -- not for free, but if you can't find 'em anywhere else I'd be glad to help.
And yes, that is where your snails have gone, but no you don't have to worry about your clown, at least not for a long while. I'm about to have to upgrade his shell again and he hasn't bothered my clown at all, except when he's eating and the fish are trying to snag a pellet or two...then he swats them away, but never really makes contact. As for the algae on your rock, I would try a sea hare; I got one and my crab didn't bother it, the only problem is that sea hare's are VERY touchy, they need consistent water parameters, good long acclimation time and supplemented food when the algae is all gone, which will probably be gone in 2 weeks or less in your 55 gallon tank. I fed mine with some Chaeto, but he may have enjoyed it TOO much as my LFS guy suggested that he could have gorged himself to death.
The other trick I found was keeping the crab well fed and using turbo snails, those seemed to last the longest (one made it almost 3 months if I remember correctly) but that's only if you spot feed the guy like twice a day...by that point you kind of have to pick your battles of constantyl buying new snails or having to do more water changes/cleaning out your protein skimmer more often.