hermit crabs exhibit new behavior

I've had red-legged hermit crabs in my 55gal tank for at least a year
now ... this is my second batch but these guys have been in there for
at least half a year.

Today I see one of them crawling up the side glass of the tank. This
side faces a corner and I allow coraline algae to grow on it. It's
like watching a Spiderman routine, but he must be somehow holding onto
the algae ... also looks like he might be feeding on it. Moving very
slowly.

So I'm thinking I'm going to post a note to this
rec.aquaria.marine.reefs describing this new behavior, and before I
head over to my PC, I look into the tank again ... another hermit crab
is doing the same thing.

No recent changes to the tank as far as I can tell. I started dripping
kalkwasser at night a couple of months ago, that's about it. The
coraline algae layer on the Spiderman wall doesn't appear to be thicker
than before, though it might be.

I've never seen my hermit crabs doing this before. Any ideas as to
what might be provoking them to do this?

Thanks in advance.