It is just detritus trapped by small algae growth... Just wipe it off... nothing serious... If it is the dying anemone den throw the anemone away...
Jizzhut, That anemone is a magnifica anemone, not the easiest anemone around, it is very light demanding and very sensitive.. I only have success with them once only... The rest will melt away.. And your doesnt look good.. You can prepare to throw it away liao..