This is just personnal experience but it may be of use. Ive found that Symbiotic Anemones will commonly do this under certain situations. most common of which is the need to exchange water that is held within the body cavity if that water has become unsuitable or unstable by way of chemistry or excessive nutrient load.
This is why its very common for anemones to deflate and re-inflate when swapped from one water body to another as a means of equalising differences in chemistry between water outside the body, and that within.
In situations where anemones are housed in systems with nutrient levels far in excess of normally accepted ranges, they will commonly try to flush themselves out more often. The worse the water quality, the more often they do it. although this trend doesn't seem to hold so true for some species commonly found in shallow water lagoon and reed flat locations that are in many cases inherently higher in dissolved nutrients anyway, such as the hells-fire, night anemones and other related species that seem better adapted to (for want of a better description) muckier conditions in comparison to those species more commonly found on the forward reef slope, in clear low nutrient water but with a high small prey item %.
Ive commonly used this trait as an indic
ator of overall welfare of symbiotic anemones with good success, and generally expect around 1 full deflation every few months as a norm (not including night time moderate deflation or retraction which is normal for most daytime species) any more than that and Id generally start looking for causes.
As for feeding, well, to be honest i never actively feed my anemones at all ( 2 bubbles, 3 Cerianthus, and an unknown night/hells-fire anemone, they just get what floats by although admittedly i do feed very heavily whist still maintaining a low free nutrient load, but i never gross feed, especially in the case of mildly stinging species such as the bubble tip because in nature they generally wont ever acquire or capture such large morsels. unlike carpet anemones that do have the capacity to capture and hold/paralyse quite sizable prey items Ive generally found that bubbles and malu's etc do better if squirted once in a while (maybe once a week or so) with a small amount of fine thawed frozen foods such as mysis or Krill Pacifica (smaller than Jumbo Krill).
It may just be a case that its being fed a little too much too often in large doses rather than smaller doses spread over a longer period.
Just a few thoughts, but looking at the image, The foot appears fine. So I'm hazarding a guess that its just changing its internal water volume.
additionally but don't quote me on this....iirc expulsion of material from the gut cavity without deflation is commonly just the anemone getting rid of non digestible material which is usually released as a kind of mucus ball. however expulsion of brown stringy material with deflation may well be the expulsion of dead or excess zooxanthallae.
Regards
Simon.
